122 EARLY STAGES IN THE 



early trace of the umbilical canal found in Birds and Selachians, 

 by which the alimentary tract is placed in communication with 

 the yolk-sac. The segmentation cavity has by this stage com- 

 pletely vanished, and the epiblast with its accompanying meso- 

 blast has spread completely round the yolk material so as to 

 form the ventral wall of the body. 



Though in some points this manner of development may 

 seem to differ from that of the Frog, there is really a funda- 

 mental agreement between the two, and between this mode of 

 development and that of the Selachians we shall find the agree- 

 ment to be very close. 



After segmentation we find that the egg of a Selachian 

 consists of two parts one of these called the germinal disc 

 or blastoderm, and the other the yolk. The former of these 

 corresponds with the epiblast and the part of the lower pole 

 composed of smaller segments in the last-described egg, and 

 the latter to the larger segments of the lower pole. This latter 

 division, owing to the quantity of yolk which it contains, has not 

 undergone segmentation, but its homology with the larger seg- 

 ments of the previous eggs is proved (i) by its containing a 

 number of nuclei (E I, n\ which become the nuclei of true cells 

 and enter the blastoderm, and (2) by the presence in it of a 

 number of lines forming a network similar to that of many cells. 

 The segmentation cavity, as before, lies completely within the 

 lower layer cells 



The next stage, E II, is almost precisely similar to the 

 second stage of the last egg. As there, the primitive invo- 

 lution is merely represented by a split separating the yolk and 

 the germinal disc, and on the dorsal side alone is there a true 

 cellular wall for this split, and at the dorsal mouth of the split 

 the alimentary epithelium becomes continuous with the epiblast. 



The segmentation cavity has become diminished, and round 

 the yolk the epiblast, accompanied by a layer of mesoblast, is 

 commencing to grow. In this growth all parts of the blasto- 

 derm take a share except that part where the epiblast and hypo- 

 blast are continuous. This manner of growth is precisely what 

 occurs in the Frog, though there it is not so easily made out ; 

 and not all the investigators who have studied the Frog have 

 understood the exact meaning of the appearances they have 



