FORMATION AND FATE OF THE PRIMITIVE STREAK. 91 



3. It is the posterior portion of the blastopore — Morgan {Ambly stoma 

 punctatum), (39), Schwarz (47), Goette (15). 



4. It is a secondary opening in the situation of the posterior part of 

 the primitive blastopore — Erlanger (10a). 



The Enclosure of the Yolk by Epiblast and the Formation 

 of the Archenteron. 



At the period generally spoken of as the end of the segmentation 

 the anural ovum is a sphere which contains an excentrically situated 

 segmentation cavity. 



The segmentation cavity has a roof which ultimately becomes the 

 anterior wall of the gastrula ; for the anus, which marks the posterior 

 end of the embryo, appears at the opposite pole of the ovum, that is 

 in the floor of the segmentation cavity. 



The roof of the cavity is formed by two or three rows of compara- 

 tively small pigment-bearing cells, and the floor by a mass of ill-defined 

 nutriment-laden cells which collectively form the yolk. At the margin 

 of the segmentation cavity the two walls merge into each other by a 

 series of insensible gradations, so that it is impossible to say where one 

 ends and the other begins. Nevertheless it is certain that the cells 

 of the roof of the segmentation cavity are epiblast, for they 

 ultimately form a portion of the external covering of the embryo ; but 

 it is incorrect to speak of the opposite wall, the yolk-cells, as modified 

 hypoblast (1, p. 103), unless it is allowed that epiblast, may be formed 

 from modified hypoblastic cells. For during the formation of the 

 blastopore the epiblast does not grow over the yolk-cells, enclosing 

 them by a process of epibolic invagination. If it did, it would be 

 possible to recognise on section a line along which the epiblast 

 terminated. No one has described or figured such a line of limitation 

 except diagrammatically ; and a careful examination of thin sections 

 of anural ova, in the stages preceding the completion of the circular 

 blastopore, shows clearly that the descriptions given of the gradual 

 extension of the epiblast over the yolk represent theory rather than fact. 



The changes observable are, firstly, that the cells of the superficial 

 layer of the yolk gradually become more distinctly pigmented ; and, 

 secondly, that they thereafter divide into smaller and more distinct 



