FOEMATION AND FATE OF THE PRIMITIVE STREAK. 95 



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 archen- 

 teron. 



2. The archenteron of the Anura is not formed by invagination, 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 

 desposition of pigment in the adjacent margins of a double row of 

 yolk cells. 



4. JSTo portion of the archenteric wall is formed by invaginated 

 epiblast ; on the contrary, the archenteron is surrounded in the first 

 phases of its development by large yolk-cells, which eventually give 

 rise to the definite hypoblast. 



5. The ventral lip of the blastopore indicates the posterior end of 

 the primitive ventral wall of the archenteron. 



6. There is no ventral and forward extension of the archenteron in 

 front of and below the ventral lip of the blastopore by the production 

 of a diverticulum in that situation. 



7. The ventral wall of the archenteron, in front of the ventral lip 

 of the blastopore, is completed by the extension of the anterior end of 

 the cavity, and by the withdrawal and modification of the cells of the 

 yolk-plug. 



Manner in ivhich the Anus of Rusconi closes. 



Having discussed the changes that take place in the ovum up to the 

 time of the formation of the large circular blastopore or anus of 

 Rusconi, we will now proceed to describe the manner in which, accor- 

 ding to our observations, the closure of the blastopore or anus of 

 Rusconi seems to be effected, and to give in some detail an account of 

 the structures which are the immediate result of the method of closure 

 about to be described. 



According to some of the former accounts, to which we have made 

 reference above, the anus of Rusconi has been said to diminish in size 

 by the gradual coming together of each portion of the blastoporic 

 rim simultaneously. This we believe to be incorrect. No doubt from 

 surface views alone the hitherto accepted accounts receive much support. 



