WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROFA. 301 



is a layer of nearly isodiametric cells, — the ectoderm, — which at one 

 point is thickened to form the germinal disk, both by an increase 

 in the diameter of the cells at right angles to the surface, and by an 

 increase in the number of cell layers. On the outside of the ectoderm 

 is found here and there a " Deckschicht " cell, apparently in process 

 of disintegration. In short, the blastodermic vesicle has seemingly 

 completed only recently the so-called first phase in mammalian gas- 

 trulation, as advocated by Hubrecht ('88 and '90) and Keibel ('89 

 and '93).- The germinal disk is slightly elliptical in outline ; not far 

 from one pole of the siiorter axis a proliferation of ectodermal cells 

 has taken place, so that three cells have come to lie above the general 

 surface of the disk. Consequently, and for additional reasons which 

 I shall give later, I consider the shorter axis to be the chief axis of 

 the embryo, and the pole where the proliferation of cells takes place 

 the posterior pole. There is, furthermore, anterior to this prolifera- 

 tion, a slight elevation at the two lateral margins of the disk, while 

 along the median line between them there is a depression. 



As the embryo develops, the germinal disk grows by a multiplication 

 of cells ; the area covered by the disk is, however, augmented slowly, 

 the tendency being, for a certain period, to an increase in thickness. 

 Thus the germinal disk, sections of which are shown in Figs. 14 

 to 21 on Plates III. and IV., is relatively much thicker than those 

 represented by Figs. 7 to 10 on Plate II. While the disk increases 

 thus in thickness, the proliferation of cells at the posterior end con- 

 tinues, producing a distinct upfolding or overgrowth in that region, 

 and at the same time a similar process has been going on at the two 

 lateral margins. Soon these three overgrowths meet and fuse, form- 

 ing one continuous bridge, at first attached at only three points, but 

 later coming in contact with the disk at all points of the margin, save 

 the anterior. There is present also a depression on the surface of the 

 ectoderm of the disk immediately beneath the bridge, and the cavity 

 which lies between this surface and the under surface of the bridge 

 is connected by a narrow canal with the cavity which surrounds the 

 disk between the extra-germinal ectoderm and the entoderm. The 

 bridge, furthermore, seems to grow not only by a proliferation of the 

 ectodermal cells of the germinal disk, but also by additions from the 

 adjacent cells of the extra-germinal area. 



There is strong evidence of such a method of development as I 

 have just traced, not only in the figures which I have reproduced 

 here, but also in the case of several embryos in which the disk is 

 much larger than at this stage, and has clearly made a greater 



