When the yolk is very much increased in amount, the nuclei
produced by the division of the zygote nucleus are unable to bring
about a surface tension sufficient to divide the cytoplasm, and so we
get a multiplication of nuclei without the formation of blastomeres.
When this happens segmentation is confined to the animal pole of
the egg and results in the formation of a thin disc of blastomeres
termed the " blastoderm," resting on an unsegmented " yolk."
Such eggs (for instance the hen's egg) are termed " meroblastic "
(gr. yuepos, a part) in contradistinction to eggs, like those of the
frog, which are completely divided and are termed " holoblastic."
In centrolecithal eggs, like those of the crayfish, the egg appears to be completely divided into cells, but although division may at first be complete, the lowered surface tension of the inner yolky ends of the blastomeres is unable to keep them apart and they flow together so as to form a common inner yolky mass. Such eggs are said to exhibit superficial segmentation. Later, the outer protoplasmic ends of these incomplete blastomeres become completely cut off, so as to form a skin of cells of blastoderm surrounding a central " yolk." A still further modification of this type is found in the eggs of insects in which the yolk is so abundant as to prevent all segmentation. The zygote nucleus alone divides and gives rise to daughter nuclei each surrounded by an island of protoplasm; these are at first dispersed throughout the " yolk " but they gradually migrate to the surface and here form a blastoderm.
In primitive alecithal eggs segmentation results in the formation of a hollow ball of cells one layer thick. This ball is termed the " blastula " and its cavity the " blastocoele," " segmentation-cav- ity " or " primary body-cavity." The formation of the blastula marks the accomplishment of an important step in development. Although typically formed only in alecithal eggs, it appears in a modified form in telolecithal eggs, even in those in which there is so much yolk that they have meroblastic segmentation. Thus in the case of the frog the blastula is a hollow ball of which the roof is two thick and the floor is many cells thick, whilst in the case of the pigeon the blastula is represented by a stage in which the blastoderm is one layer thick and forms the roof and is separated by a slit-like cavity from the immense mass of the unsegmented yolk forming the floor in the uppermost layer of which are a few nuclei. These nuclei are representatives of the cells which should constitute the vegetative pole of the blastula but they are utterly unable to cut the yolk up into cells.
Formation of Germ Layers. As soon as the blastula stage has been attained, the " formation of layers " begins. The cells at the vegetative pole become turned inwards, forming a tube-like structure which projects into the blastocoele and partially obliterates it. This tube is the primitive gut or " archenteron " and the cells form- ing it are termed " endoderm," whereas the cells forming the outer wall of the blastula give rise to the primitive skin and are termed " ectoderm." Driesch 1 has shown that until the archenteron begins to be formed all the cells of the blastula of Echinus are alike in their potencies; any sufficiently large piece of it, if cut off, will round itself off and form a blastula and ultimately a perfect larva of diminished size; after a region has been delimited as the centre of the formation of the endoderm the rest of the blastula wall, if cut off, can no longer form an archenteron and so it follows that when the endoderm is differentiated at one place, the rest of the blastular wall becomes changed into definitive ectoderm.
When the archenteron has been formed the developing egg has assumed the shape of a double-walled cup, the opening into which is termed the " blastopore." This stage is clearly and sharply marked in the development of almost all eggs in which the yolk is small in amount, and it can be recognized in an obscured and altered form in the development of large yolky eggs. It is of equal importance to the blastula stage, and it is termed the " gastrula."
The primary body-cavity has now become reduced to the slit intervening between the wall of the archenteron and the outer wall of the gastrula and this slit becomes largely filled up by the de- velopment of the third germ layer, the " mesoderm." We have defined this layer as the primitive peritoneum or lining of the body- cavity, but the body-ca'vity now indicated is termed the " coelom " or " secondary body-cavity " in order to distinguish it from the primary body-cavity. In the eggs of primitive animals, where the yolk is small in amount, the coelom is always formed as a series of pouch-like outgrowths of the archenteron which become cut off from this tube. It follows that the mesoderm is differentiated from the primary endoderm. Driesch 2 has shown that if the front half of the gastrula of the starfish which includes the apex of the archenteron be cut off, the hinder half will heal up and will form a perfect larva, forming, of course, the coelom in the normal way. If, however, this operation be performed after a swelling of the tip of the archenteron the first rudiment of the coelom has appeared, then, although the hinder half will heal up and form a larva, it never forms a coelom. Driesch concludes from this experiment that at first all parts of the archenteric wall have the power of giving rise to a coelom, that is of forming mesoderm, but that later a definite portion of this wall becomes set aside as the rudiment of the coelom and that then
1 H. Driesch, " Zur Analysis der Potenzen embryonaler Organ- zellen," Archiv fur Entwicklungsmechanik, vol. ii., 1896.
2 loc. cit., p. 20.
the rest of it becomes the definitive endoderm devoid of this coelom- forming power. In Echinodermata the coelom arises as a single pouch from the apex of the archenteron ; in primitive Vertebrata it originates as five pouches of which one is apical and four are paired and lateral ; in Chaetognatha and Brachiopoda as a lateral pair of pouches. The remnant of the primary body-cavity becomes almost filled up with cells budded from the wall of the coelom which are termed " mesenchyme." These cells may become joined to one another by their processes and thus constitute a network which becomes converted into connective tissue by the secretion of fibres; or they may remain separate from one another, and then they become developed into blood and lymph cells, the remnants of the primary body-cavity constituting the blood-spaces. In the Coelenterata, in which no coelom is formed, similar cells are budded from both ecto- derm and endoderm; in Annelida and Mollusca, in addition to the mesenchyme given off from the coelomic wall, some is likewise budded from the ectoderm, and to this the name " mesectoderm " has been given. In Vertebrata the most recent research indicates that no mesenchyme is given off from the ectoderm.
Organogeny. Turning now ,to the third stage of development, viz. the formation of special organs, we find that from the ectoderm are derived the central nervous system and the sense organs, and also the lining of the mouth-cavity and of the terminal portion of the alimentary canal near the anus. The endoderm gives rise to the middle portion of the gut and to the glands which are developed from it, and in Vertebrata to the primitive elastic axis of the back-bone or " notochord." From the mesoderm arise the majority of the muscles, the connective tissue, and, in Vertebrata and Echinoder- mata, the internal calcareous skeleton which is derived from the con- nective tissue. The mesoderm also gives rise to the genital organs and their ducts in all Metazoa above the rank of Coelenterata and in Mollusca and Vertebrata to the kidney tubules.
Now we have pointed out that, in telolecithal eggs, segmentation proceeds most rapidly at the animal pole; here the second stage of development rapidly supervenes, and the archenteron is begun before segmentation is even initiated at the vegetative pole. In meroblastic eggs the upper pole of the egg may become converted into an embryo in which all the important organs of the adult are mapped out before the lower pole is even invested with cells. Finally in Amniota (reptiles, birds, and mammals) the lower pole of the egg, after all the yolk has been absorbed from it, is torn from the rest of the embryo at birth and cast off as a useless embryonic membrane.
In the earlier article a strong attempt was made to show that the primitive germ-layers do not correspond to one another in different eggs ; in a word, that the same name has been given to different things.
Some of the arguments adduced are the diverse origins of the mesoderm in various animals, and the alleged origin of the epithe- lium of the alimentary canal of insects and some other Arthropoda from the ectoderm. The result of the labours of embryologists during the last 15 years has been to establish the universal homology of the germ-layers on an ever firmer basis, and to show that the difficulties alluded to were based on faulty observations.
If, for instance, we define the mesoderm as the wall of the coelom then it is found that this organ originates in one of two ways, viz. : either as a pouch or a mass of cells. The pouch (recognizable in Chaetognatha, Brachiopoda, Echinodermata, Enteropneusta and the lowest Vertebrata) quite clearly originates as an outgrowth from the endoderm ; the mass of cells can be traced back to its source in one large cell, the mother mesoderm-cell. This cell, as was first shown by Shearer 3 in the annelid Hydroides and by Con- klin 4 in the mollusc Crepidula, originally forms part of the wall of the archenteron and its ejection from this wall is evidently a modifica- tion of the more primitive method of coelom-formation by the outgrowth of a gut-pouch. Attempts which have been made by Meissenheimer 5 and Harms 6 to show that in Mollusca the coelom originates from cells budded from the ectoderm are based on ob- vious blunders in missing out stages in reconstructing the life-his- tory that most fertile source of error in embryology. Later workers have exposed this error and have shown that in the Mollusca, with which Meissenheimer and Harms dealt, the mother mesoderm cell gives rise to the pericardium which is representative of the coelom in these animals. We have already alluded to the presence of mesectoderm in Annelida and Mollusca; this gives rise to some superficial muscles, but to confound this with the coelomic wall and its derivatives by calling both mesoderm and then to complain that the mesoderm is not an homologous structure in various groups of animals is to introduce a perfectly gratuitous confusion.
We may now turn to the alleged origin of the gut epithelium of certain Arthropoda from the ectoderm. In the earlier article the state- ment was made that in the embryo of that most primitive of all land Arthropoda Peripatus there is a large slit-like blastopore which later
8 C. Shearer, " On the development and structure of the Tro- chophore of Hydroides," Quart. Journ. Micr. Sci.,\o\. xiii. (N.S.), 1911.
4 Conklin, " The Embryology of Crepidula," Journal of Mor- phology, vol. xiii., 1897.
6 Meissenheimer, " Entwicklungsgeschichte yon Dreissensia poly- morpha," Zeitschrift f. wissenschaftliche Zoologie, vol. Ixix., 1901.
6 Harms, " Postembryonale Entwicklungsgeschichte der Unioni- der," Zoo/. Jahrbucher (Abt. fur Ontogenie), vol. xxviii., 1909.
