Development of the Second Day 141 



medullary canal, except for a small chink at 

 the extreme end. 



The anterior end of the medullary canal 

 now begins to enlarge slightly and to be con- 

 stricted off from the rest of the tube (Fig. 

 47) this enlargement is the beginning of the 

 fore-brain, and from each side of it there 

 is soon seen a small lateral diverticulum, the 

 optic vesicle, whose further development will 

 be described later. 



Behind the fore-brain two other slight con- 

 strictions of the neural canal are formed (Fig. 

 47) marking the positions of the mid-brain, 

 V 2, and the hind-brain, V $. At the pos- 

 terior end of the embryo the medullary folds 

 are still some distance apart, forming what 

 is known as the sinus rhomboidalis (Fig. 47, 

 ^ R), along the floor of which may sometimes 

 be seen the primitive streak, pp. 



The beginning of the ears may be seen as 

 a pair of small depressions, the auditory pits, 

 just back of the hind-brain (Fig. 47, V A). 



The mesoblastic somites have increased in 

 number, and by the thirty-sixth hour, as has 

 already been mentioned, there are usually 

 fifteen pairs of them (Fig. 47, P V). 



The head is now more clearly defined, owing 



