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 tnid-brain, 

 V 2, and the hind-brain, V 3. 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, 

 6" K), 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 



