ELASMOBRANCHII. 47 



somites. The notochord, which was formed long before the stage repre- 

 sented 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 alimen- 

 tary tracts become continuous 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 {al) 

 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 outwards, 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 progressed, 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 fore- 

 most of these the optic vesicles now present themselves as well- 

 marked lateral outgrow^ths, 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 Qit) becomes visible during this stage as a cavity between the 

 mesoblast of the splanchnopleure and the hypoblast. 



The fore and hind guts are now longer than they were. An inva- 

 gination from the exterior to form the mouth has appeared (m) on 

 the ventral side of the head close to the base of the thalamence- 

 phalon. The upper end of this eventually becomes constricted off as 

 the pituitary body, and an indication of the future position of the 

 anus is afforded by a slight diverticulum of the hind gut towards the 

 exterior, some little distance from the posterior end of the embryo 

 {an). The portion of the alimentary canal behind this point, though 

 at this stage large, and even dilated into a vesicle at its posterior end 

 {al.v), becomes eventually completely atrophied. It is known as 

 the postanal gut. In the region of the throat the rudiment of a 



