EMBRYOLOGV 



481 



vacuolated. Its front end grows forwards to the end of the 

 snout. The hind end is for a long time connected with the 

 hypoblast in front of the neurenteric canal. The mesoderm 

 arises as five hollow outgrowths (Figs. 360, 361). One of 

 these is median and unpaired in front. Behind it lies a pair 

 of dorso-lateral pouches at the sides of the notochord, and 

 behind these two long dorso-lateral grooves. The median 

 and first two lateral pouches soon separate from the gut, 

 though the left one retains a narrow connection for some 

 time, but the grooves, as fast as they close off in front, are 

 prolonged backwards. As growth progresses the separated 



nek. 



Fig. 360. — Transverse sections of embryos of Amphioxus at two stages 

 somewhat later than that represented by 11 (Fig. 354), showing the 

 origin of the nerve cord, notochord, and mesoblast. — After Hatschek. 



ach., Archenteron ; mes.p., pouch which will become first mesoblastic somite ; 

 n.p. t neural plate ; uc/t., rudiment of notochord. 



anterior part of the groove becomes segmented into a series 

 of pouches like the first pair. These pouches are the 

 mesoblastic somites. They will presently spread between 

 ectoderm and endoderm and give rise to the mesoderm of 

 the adult. It will be seen that the "endoderm" or "hypo- 

 blast " which forms the lining of the gastrula is a layer which 

 gives rise not only to the hypoblast of the larva (which 

 becomes the endoderm of the adult) but also to mesoblast 

 (which becomes mesoderm). For this reason we may call 

 it the primitive hypoblast. In the same way, the archenteron 

 gives rise both to the enteron and to the hollows of the 

 mesoblastic somites. The rudiments of all three layers of a 



