ELASMOBRANCHII. 57 



before the stage represented in figure 28 D, is now also distinctly 

 visible. It extends from almost the extreme posterior to the 

 anterior end of the embryo, and lies between the ventral wall of 

 the spinal canal and the dorsal wall of the intestine. Round its 

 posterior end the neural and alimentary tracts become continu- 

 ous with each other. Anteriorly the termination of the 

 notochord cannot be seen, it can only be traced into a mass of 

 mesoblast at the base of the brain, which there separates the 

 epiblast from the hypoblast. The alimentary canal (ai] is 

 completely closed anteriorly and posteriorly, though still widely 

 open to the yolk-sack in the middle part of its course. In the 

 region of the head it exhibits on each side a slight bulging out- 

 wards, the rudiment of the first visceral cleft. This is 

 represented in the figure by two lines (I. v.c.}. 



The embryo represented in fig. 28 E is far larger than the 

 one just described, but it has not been convenient to represent 

 this increase of size in the figure. Accompanying this increase 

 in size, the folding off from the yolk has considerably pro- 

 gressed, and the stalk which unites the embryo with the yolk is 

 proportionately narrower and longer than before. 



The brain is now very distinctly divided into the three lobes, 

 the rudiments of which appeared during the last stage. From 

 the foremost of these the optic vesicles now present themselves 

 as well-marked lateral outgrowths, towards which there has 

 appeared an involution from the external skin (op} to form the 

 lens. 



A fresh organ of sense, the auditory sack, now for the first 

 time becomes visible as a shallow pit in the external skin on 

 each side of the hind-brain (au.v). The epiblast which is 

 involuted to form this pit becomes much thickened, and thereby 

 the opacity, indicated in the figure, is produced. 



The mesoblastic somites have greatly increased in number 

 by the formation of fresh somites in the tail. Thirty-eight of 

 them were present in the embryo figured. The mesoblast at 

 the base of the brain is more bulky, and there is still a mass of 

 unsegmented mesoblast which forms the tail swellings. The 

 first rudiment of the heart (Jit] becomes visible during this stage 

 as a cavity between the mesoblast of the splanchnopleure and 

 the hypoblast. 



