stage and gives rise to the muscular bands of the young larva. In
Echinodermata a certain amount of mesenchyme appears before the
epithelial mesoderm, which is formed later as gut-diverticula. In
these forms the mesenchyme is said to arise as wandering amoeboid
cells, which are budded into the blastocoel by the endoderm just
before and during its invagination, but the writer has reason to
believe that this account of it does not quite describe what happens.
It would seem to be more probable that the mesenchyme arises in
these forms, as it certainly does in the case of the later-formed
mesenchyme of the Vertebrate embryo, as a protoplasmic outflow
from its tissue of origin, passing at first along the line of pre-existent
protoplasmic strands which traverse the blastocoel, and sending
out at the same time processes which branch and anastomose with
neighbouring processes (see E. W. MacBride, Proc. Camb. Phil. Soc.,
1896, p. 153). In the Vertebrata the whole of the mesoderm has at
first the mesenchyme form. Afterwards, when the body-cavity split
appears, the bulk of it assumes a kind of modified epithelial condition,
which later on yields, by a process of outflow very similar in its
character to what has been supposed to occur in the Echinoderm
blastula, a considerable mesenchyme of the reticulate character.
Mesenchyme is the tissue which in Vertebrate embryology has frequently
been called embryonic connective tissue. This name is no
doubt due to the fact that it was supposed to consist of isolated
stellate cells. It is, however, in no sense of the word connective
tissue, because it gives rise to many organs having nothing whatever
to do with connective tissue. For instance, in Vertebrata this tissue
gives rise to nervous tissue, blood-vessels, renal tubules, smooth
muscular fibres, and other structures, as well as to connective and
skeletal tissues. The Vertebrata, indeed, are remarkable for the
fact that the epithelial tissues of the so-called mesoderm, e.g. the
epithelial lining of the body-cavity, and of the renal tubules and
urogenital tracts, all pass through the mesenchymatous condition,
whereas in Amphioxus, Balanoglossus and presumably Sagitta and
the Brachiopoda, all the mesodermal tissues pass through the
epithelial condition, most of the mesodermal tissues of the adult
retaining this condition permanently. As has been implied in the
above account, mesenchyme is usually formed from epithelial
mesoderm or from endoderm, or from tissue destined to form
endoderm. It is also sometimes formed from ectoderm, as in the
Vertebrata at the nerve crest and other places. In some Coelenterata
also it appears certain that the ectoderm does furnish tissue of a
mesenchymatous nature which passes into the jelly, but this phenomenon
takes place comparatively late in life, at any rate after the
embryonic period. In this connexion it may be interesting to point
out that in many Coelenterates all the tissues of the body retain
throughout life the epithelial condition, nothing comparable to
mesenchyme ever being formed.
Finally, before leaving this branch of the subject, the fact
that the three germinal layers are continuous with one another,
and not isolated masses of tissue, may be emphasized.
Indeed, an embryo may be defined as a multinucleated
protoplasmic mass, in which the protoplasm at any
Continuity of
the layers.
surface—whether internal or external—is in the form
of a relatively dense layer, while that in the interior is much
vacuolated and reduced to a more or less sparse reticulum, the
nuclei either being exclusively found in the surface protoplasm,
or if the embryo has any bulk and the internal reticulum is at
all well developed, at the nodes of the internal reticulum as well.
The origin of some of the more important organs may now be considered. It is a remarkable fact that the mouth and anus develop in the most diverse ways in different groups, but as a rule either one or both of them can be traced into relation with the blastopore, the history of which Mouth and anus. must therefore be examined. In most, if not all, the great groups of the animal kingdom, e.g. in Coelenterata, Annelida, Mollusca, Vertebrata, and in Arthropoda, the blastopore or its representative is placed on the neural surface of the body, and, as will be shown later on, within the limits of the central nerve rudiment. Here it undergoes the most diverse fate, even in members of the same group. For instance, in Peripatus capensis it extends as a slit along the ventral surface, which closes up in the middle, but remains open at the two ends as the permanent mouth and anus. In other Arthropods, though full details have not yet in all cases been worked out, the following general statement may be made:—A blastopore (certain Crustacea) or its representative is formed on the neural surface of the embryo and always becomes closed, the mouth and anus arising as independent perforations later. Here no one would doubt the homology of the mouth and anus throughout the group; yet within the limits of a single genus—Peripatus—they show the most diverse modes of development. In Annelids the blastopore sometimes becomes the mouth (most Chaetopoda); sometimes it becomes the anus (Serpula); sometimes it closes up, giving rise to neither, though in this case it may assume the form of a long slit along the ventral surface before disappearing. In Mollusca its fate presents the same variations as in Annelida. Now in these groups no zoologist would deny the homology of the mouth and anus in the different forms, and yet how very different is their history even in closely allied animals. How are these apparently diverse facts to be reconciled? The only satisfactory explanation which has been offered (Sedgwick, Quart. J. Mic. Science, xxiv., 1884, p. 43) is that the blastopore is homologous in all the groups mentioned, and is the representative of the original single opening into the enteric cavity, such as at present characterizes the Coelenterata. From it the mouth and anus have been derived, as is indicated by its history in Peripatus capensis, and by the variability in its behaviour in closely allied forms; such variability in its subsequent history is due to its specialization as a larval organ, as a result of which it has lost its capacity to give rise to both mouth and anus, and sometimes to either.
That the blastopore does become specialized as a larval organ is obvious in those cases in which it becomes transformed into the single opening with which some larvae are, for a time at least, alone provided, e.g. Pilidium, Echinoderm larvae, &c., and that larval characters have been the principal causes of the form of embryonic characters, strong reason to believe will be adduced later on. In the Vertebrata the behaviour of the blastopore (anus of Rusconi) is also variable in a very remarkable manner. As a rule it is slit-like in form and closes completely, but in most cases one portion of it remains open longer than the rest, as the neurenteric canal. In a few forms (e.g. Newt, Lepidosiren, &c.) the very hindermost portion of the slit-like blastopore remains permanently open as the anus, and from such cases it can be shown that the neurenteric aperture (when present) is derived from a portion of the blastopore just anterior to its hindermost end. The words “hindermost” and “anterior” are used on the assumption that the whole blastopore has retained its dorsal position; as a matter of fact the hindermost part of it—the part which persists or reopens as the anus—loses this position in the course of development and becomes shifted on to the ventral surface. This is clearly seen in Lepidosiren (Kerr, Phil. Trans. cxcii., 1900), in Elasmobranchii, and in Amniota (primitive streak). Moreover, in Lepidosiren, and possibly in some other forms, the anus, i.e. the hind end of the blastopore, is at first contained within the medullary plate and bounded behind by the medullary folds. Later the portions of the medullary plate in the neighbourhood of the anus completely atrophy, and this relation is lost. This extension of the hind end of the blastopore on to the ventral surface, and atrophy of the portion of the medullary plate in relation with it, is a highly important phenomenon, and one to which attention will be again called when the relation of the mouth to the blastopore is being considered. The remarkable fact about the Vertebrata, a feature which that group shares in common with all other Chordata (Amphioxus, Tunicata, Enteropneusta) and with the Echinodermata, is that the mouth has never been traced into relation with the blastopore. For this reason, among others, it has been held by some zoologists that the mouth of the Vertebrata is not homologous with the mouth of such groups as the Annelida, Arthropoda and Mollusca. But, as has been explained above, in face of the extraordinary variability in the history of the mouth and anus in these groups, this view cannot be regarded as in any way established. On the contrary, there are distinct reasons for thinking that the Vertebrate mouth is a derivate of the blastopore. In the first place, in Elasmobranchii (Sedgwick, Quart. Journ. Mic. Sci. xxxiii., 1892, p. 559), and in a less conspicuous form in other vertebrate groups, the mouth has at first a slit-like form, extending from the anterior end of the central nerve-tube backwards along the ventral surface of the anterior part of the embryo. This slit-like rudiment, recalling as it does the form which the blastopore assumes in so many groups and in many Vertebrata, does suggest the view that possibly the mouth of the Vertebrata may in reality be derived from a portion of an originally long slit-like neural blastopore, which has become extended anteriorly on to the ventral surface and has lost its original relation to the nerve rudiment, as has undoubtedly happened with the posterior part, which persists as the anus.
Of the other organs which develop from the two primary layers it is only possible to notice here the central nervous system. This in almost all animals develops from the ectoderm. In Cephalopods among Mollusca—the development of which is remarkable from the almost Central nervous system. complete absence of features which are supposed to have an ancestral significance—and in one or two other forms, it has been said to develop from the mesoderm; but apart from
