180 ARTHUR E. SHIPLEY. 



in older embryos. A similar change in the cells lining the 

 mesenteron takes place at its posterior end. The cells lining 

 the blastopore and extending for some distance into the ali- 

 mentary canal assume very early a columnar appearance and 

 appear perfectly continuous with the columnar epiblast (figs. 

 10, 14, and 16.) The cells lining the hind-gut retain the 

 character of the yolk-cells for a long time, but the lumen of 

 the mesenteron in this region never disappears, as Scott and 

 Calberla thought. The lumen of the alimentary canal, with 

 the exception of the mouth, is derived directly from the inva- 

 gination which forms the gastrula, and no part of it is ever 

 obliterated in the course of development. 



A similar persistence of the blastopore to form the anus appears 

 to be common in the Amphibia. It has been shown to occur 

 in the Newt by Miss Johnson, in the Frog by Spencer, 

 and in Alytes by Gasser. Its occurrence in the Cyclos- 

 tomata seems to point to the fact that it is a primitive 

 feature retained in those eggs whose development is not greatly 

 modified by the presence of a large mass of yolk. Renewed 

 observations in the development of Amphioxus would pro- 

 bably throw some light on this point. 



The Central Nervous System. 



The early development of the central nervous system has 

 been so fully described by Calberla, Balfour, and Scott, 

 that little is left to be added to their account. But the origin of 

 the neural canal, the relationship of the posterior end of the neural 

 cord to the blastopore, and the later development of the parts 

 of the brain and the cranial nerves present points of interest. 



Calberla was the first to show that the central nervous 

 system of the Lamprey arises by a delamination and not by 

 an involution of the epiblast. He described a similar origin for 

 the nervous system of the Teleostei, and Balfour and 

 Parker found the same to be the case in Lepidosteus. 



The first trace of the neural plate appears about the eighth day 

 after fertilization, just after the invagination is completed. A 



