FORMATION AND FATE OF THE PRIMITIVE STREAK. 461 



already referred to concerning the formation of the anus in 

 Amphibians. 



1 . It is a new formation in the region below the blastopore — 

 Balfour (1), Sidebotham (51). 



2. It is the blastopore — Sedgwick (49), Alice Johnson (27), 

 Spencer (52). 



3. It is the posterior portion of the blastopore — Morgan 

 (Amblystoma 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 seg- 

 mentation 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 

 comparatively 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 ulti- 

 mately 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 dia- 



