Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/435

ANIMAL.] REPRODUCTION 417 of the " polar disk " alone exhibiting a vertical striation, and differentiating into two layers, superficial and subjacent (termed achromophilous and chromophilous respectively). The subjacent vitellus is distinguished into several layers, (central, intermediate, and cortical), and contains yolk elements of three distinct kinds, while the enveloping protoplasm has a distinctly reticulated and 'fibrillar structure, like the stroma of so many animal and vegetable cells. The germinal vesicle is practically homogeneous save that the portion surrounding its eccentric germinal spot is distinguished into an investing portion or spherical envelope, the " prothyalo- soma," and an " accessory portion," which usually contains one or two " pseudo-nucleoli. " In the ripe egg Van Beneden describes new complexities within the germinal spot ; this consists of two juxtaposed quadrilateral disks, each containing four chromatin globules, united by a substance having less affinity for colouring matter. Kadiating from these two disks, achromatin threads arise in the prothyalosoma, but stress is laid on the fact that no grouping of the chromatin elements into a star-like figure takes place. The spherical shape of the germinal spot is now modified by the intrusion on each side of a large homogeneous droplet from the vitellus into the prothyalosoma, so that in optical section it comes to have a T-shape, the accessory portion being mainly compressed to form the stalk of the T. At this stage the spermatozoon usually commences to work its way into the ovum, but does not yet affect the germinal vesicle or germinal spot, which proceed to the formation of polar globules. The T-shaped gradually passes into the "ypsiliform " figure, so called from its resemblance to the Greek T. Its steadily diverging branches, which are formed from the prothyalosoma, move upwards till they reach the surface of the vitellus, their librillar structure already noted meanwhile becoming well marked. Each bundle bears one of the two groups of four chromatiu globules which compose the germinal spot. FIG. 11. Fecundation of ovum (after Van Beneden). A, ovum before entrance of spermatozoon (showing complexity of structure) ; B, deformation of ger- minal vesicle on penetration of spermatozoon s; C, prothyalosoma with two chromatin disks and axial fibres; D, E,F, later stages of the ypsiliform figure; G, second pseudo-karyokinetic figure ; H. I, J, K, expulsion of first polar globule. Next the vertical branch of the ypsiliform figure swings upwards to the surface, and a new branch is formed as a continuation of the same line ; the whole figure is thus cross-shaped, with the prothyalosoma in the centre, but this cross soon disappears, leaving the prothyalosoma with its two chromatin groups. These are now divided, by a plane tangential to the surface of the vitellus, into two equal parts, and the upper of these, containing of course half the prothyalosoma with half of each of the two chromatin lisks, becomes the first polar globule. Van Beneden lays great stress on the fact that this plane of division is not transverse to the oblique spindle formed by the diverging branches of the T, as was to be expected from the views of all previous observers, but on the contrary is parallel to it. The remaining portion of the prothyalosoma with its two Chromatin disks, together with the surrounding protoplasm, now proceeds to the separation of the second polar vesicle. Despite differences in detail, the essential facts are the same : a spindle nth a star-like figure at each end is formed; this at first lies radially m the vitellus, but afterwards becomes superficial, and its division takes place as before by an axial not an equatorial plane. 1 late - r ancl also elaborate discussion is due to Sabatier While combating the peculiar views suggested by Weismann and others as to the polar globules of Insects (as that they re-enter the ovum to form the rudiments of the future repro- ductive organs), he admits that they may be emitted at both poles of the ovum, and may either break up, be reabsorbed, or even in some cases form peculiar structures surrounding the ovum(e.<7., the follicular cells ol Ascidians). He describes in Buccinum, &c., the extrusion of somewhat amcsboid masses of clear protoplasm at several distinct points and frequently without the appearance of any nuclear spindle ; and these may even re- peatedly divide. He holds that the centri- fugal extrusion of elements from ova is much more general than is usually recognized, and Fl - 12 (after Sabatier). distinguishes these into three kinds :-(!) ?**% "globules precoces,' which usually go to O f polar globules and the form the elements of the follicle ; (2) "glob- associated elevation of ules tardifs," which appear later; and (3) clear protoplasm, true polar vesicles, which alone are associated with karyokinetic changes of the nucleus. Like Van Beneden, he notes that polar vesicle formation is not strictly comparable to ordinary cell-division in being not trans- verse but longitudinal to the nuclear spindle. From this point little advance has been made, though careful reviews of the subject are due to Flemming, J. T. Cunningham, and others. (13) Fertilization or Impregnation of the Ovum. The funda- mental generalization now so familiar that the process of fertilization for plant and animal alike lies in the material union of both sexual products although said to have been propounded by Alcmaeon (580 B.C.), and even described by Hartsoeker (1750), is pointed out by Hensen to be essentially due in the first place to the experimental re- searches on plant hybridization of Kolreuter (1761). Jacobi soon afterwards artificially fertilized the eggs of Trout and Salmon, but the most important and really convincing work was that of Spallanzani (1780), who experimented on the Frog, Tortoise, and Bitch. He unfortunately, however, concluded that spermatozoa might be absent without pre- venting fertilization, and ascribed all fertilizing powers to their fluid medium, so establishing an error which required for its elimination many successive researches. Provost and Dumas (1824) showed that filtration really deprived the seminal fluid of its powers. Martin Barry (1843) actually observed spermatozoa within the zona pellucida of the Rabbit's ovum ; Leuckart (1849) repeated both pre- ceding observations in the Frog; Nelson (1852) observed the entrance of the spermatozoa into the ovum of Ascaris ; while Keber (1853) discovered the micropyle in the ovum of the Mussel, and watched the passage of the spermatozoon through it into the yolk. It only remained for these results to be confirmed by the high authority of Bischoff and Allen Thomson (1854); and the knowledge of the subject thus reached its second stage, where it practically remained for nearly twenty years. Aided by the advance of histological technique, a new plane was reached (1875-76) by the brilliant researches of Van Beneden on the ovum of the Rabbit, and of O. Hertwig and Fol on Echinoderm ova. The spermato- zoon 1 was thus shown not to disappear into the yolk, but to form from its head or nucleus the " male pronucleus," which meets and fuses with the "female pronucleus," as the germinal vesicle is termed, after the extrusion of the polar body, This new " conjugation-nucleus " soon ex- hibits karyokinetic changes and divides, and the segmenta- tion of the ovum rapidly progresses. As a full account of these and other papers up to 1880 is given by Balfour (Embryology, vol. i.), it will suffice to note the more important subsequent researches, especially as those, though with 1 That, though more than one spermatozoon may pass through the vitelline membrane, only one normally enters the vitellus and becomes a male pronucleus has been well made out by Hertwig, Fol, and others. In the rare cases where more spermatozoa than one force an entrance, Fol has observed the monstrous double segmentation of the ovum, and argues forcibly for the hypothesis that we have to look in this process of " polyspermy " for the explanation of numer- ous teratological and pathological changes. XX. - 53