FORMATION AND FATE OF THE PRIMITIVE STREAK. 465 



reaches the region of the blastoporic margin ; and it is this fact, 

 together ^^ith a predisposition to discover, if possible, an invagi- 

 native process, which seems to have led to the description of 

 the formation of the archenteron of the Anura by invagination 

 of the epiblast. 



A careful examination of the walls of the extending archen- 

 teric cavity reveals no evidence in support of this ideal invagina- 

 tive process ; on the contrary, it shows that even at the period 

 of completion of the blastoporic opening (see figs. 1 and 3) 

 the greater part of the cavity is surrounded by large nutri- 

 ment-laden, marginally pigmented cells. More especially is 

 this the case in the anterior portion of the, as yet, incom- 

 plete cavity (fig. 2), and at its posterior extremity (fig. 3), 

 where the slit-like space is only just formed. In the 

 latter situation the only cell that can be considered as dis- 

 tinctly epiblastic is that marked c. The remaining cells are 

 undoubtedly large yolk-cells ; and it is only by the division of 

 these cells, and the arrangement of some or all of their 

 descendants into a distinct layer, that the true hypoblastic 

 lining of the alimentary canal and its diverticula is formed. 

 As the completion of the definite hypoblast is intimately asso- 

 ciated with the separation of the mesoblast and chorda, its 

 further history cannot at this period be entered upon, and we 

 may therefore proceed to the termination of this section by a 

 summary of the conclusions deduced from that which has been 

 here set forth. 



1. The blastopore is a deficiency in the posterior wall of 

 the archenteron. 



2. The archenteron of the Anura is not formed by inva- 

 gination, but by a process of splitting amongst the yolk- 

 cells very similar to that described by Houssay (26) in the 

 Axolotl. 



3. The situation of the archenteric cavity is first defined by 

 the deposition of pigment in the adjacent margins of a double 

 row of yolk-cells. 



4. No portion of the archenteric wall is formed by iuvagi- 

 nated epiblast; on the contrary, the archenteron is surrounded 



VOL. XXXII, PART IV. NEW SEB. 1 I 



