348 AETHUR E. SHIPLEY. 



larva, fifty-two days (fig. 43). The further development of 

 the Lamprey's skull has been described by Professor Parker ia 

 his great work on ^The Skeleton of the Massipobranch 

 Fishes/ 



The Mesenteron. 



The cavity of the alimentary is formed by the invagination 

 of the endoderm described in the first section of this article, 

 when once found it does not disappear again, although in the 

 region of the intestine it may be reduced to a slit by the 

 pressure of the surrounding yolk-cells. 



The most anterior section, including the branchial region 

 and that part of the intestine in front of the liver, is now 

 separated from the rest by the raising of the head and neck 

 from the remaining part of the embryo. The lining cells of 

 this portion at once assume a columnar character ; the hypo- 

 blastic cells in the region of the blastopore, or as it may now 

 be termed the anus, also assume a similar form. But the cells 

 in the middle part of the intestine still retain the features 

 of the yolk-cells, those forming the roof of the enteron being 

 however, rather more columnar than those of the floor and 

 sides. 



In the head region almost the whole of the space inside the 

 epiblast is taken up with the brain, which has a great depth, 

 and with the notochord and the alimentary canal, which ends 

 blindly in front. A small band of mesoblast lies on each side 

 of the nervous system and notochord. This segments dorsally 

 into a series of myomeres, the first lying close behind the ear. 

 Ventrally the mesoblast has not grown down between the en- 

 doderm, so that along the sides and under surface the hypoblast 

 and epiblast are in contact. The first gill-slit appears, as Scott 

 has described, about the twelfth or thirteenth day, the others 

 arise eduring the next three or four days, the most posterior 

 being the last formed. The gill-slits appear to me to be the 

 result of the ventral downgrowth of mesoblast taking place 

 only at certain places, these forming the gill-bars. Between 

 each downgrowth the hypoblastic lining of the alimentary 



