EMBR YONIC DE VELOPMENT. I I 5 



ming existence, the embryo keeps rigidly to the surface 

 of the water. 



After its escape from the vitelline membrane, it grows 

 rapidly in length. Fresh archenteric pouches are added 

 to those already formed, one after the other, in metameric 

 order. The medullary plate (i.e. the fore-cast of the nerve- 

 tube) becomes completely closed in beneath the superficial 

 ectoderm except at its anterior extremity, where it remains 

 open to the exterior in the mid-dorsal line by an aperture 

 known as the ncuropore (Fig. 63 A, B, C). Finally, the 

 notochord becomes differentiated from the primitive endo- 

 derm. 



According to Hatschek the number of mesodennic 

 somites which arise as diverticula from the archenteron 

 is fourteen pairs. Those which are subsequently added 

 to these arise at the hinder end of the body by prolifera- 

 tion from the cells which lie behind, and at the sides of 

 the neurenteric canal, or in that region, so that they never 

 appear as actual outgrowths from the archenteron. 2 



In Fig. 63 C the embryo has undergone some radical 

 changes in form. Its body, previously cylindrical, has 

 become laterally compressed, the ectoderm cells of the 

 hinder end of the body have begun to elongate so as to 

 form the rudiment of a provisional caudal fin, and the 

 front end of the body has grown out into the shape of 

 a snout. In connexion with the latter there are two 

 remarkable structures which arise as a pair of outgrowths 

 from the anterior region of the archenteron, and were first 

 described by Hatschek as a pair of anterior intestinal 

 diverticula. These we shall return to later. 



Near the front end of the alimentary canal a curious 

 sac-like structure has appeared (Fig. 63 C}. It arose as 

 a transverse groove in the floor of the gut in the region 



