THE THIRD WEEK, 



489 



it is not yet certain whether it is to be regarded as a normal 

 feature. His has suggested that it may possibly be caused by 

 the embryo tending to increase in length more rapidly than the 

 closely fitting amnion will allow it to ; the embryo consequently 

 becoming bent at the place where its ventral wall is weakest, i.e. 

 opposite the yolk-sac. 



In the head, cranial flexure is well marked, the anterior part 

 of the head being bent down at right angles to the hinder part, 



BM 



Fig. 197. — Human Embryo lettered by Professor His, Lg, and estimated as 

 fifteen days old. The wall of the blastodermic vesicle has been removed, 

 except the portion with which the allantoic stalk is continuous. (From 

 His.) x 30. 



AN, inner or true amnion. BM, mid-brain. EI, auditory pit. HC.l, first 

 branchial cleft. EM, hyomanrtibular cleft. M!N\ mandibular arch. E, heart. TL, 

 tail. TZ, allantoic stalk. V!N", villi of chorion. YS, yolk-sac. 



and the fore-brain being in consequence carried round to the under 

 surface of the head. The prominent angle of the brain is formed 

 by the mid-brain, bm, behind which comes the nearly straight 

 hind-brain. 



At the sides of the fore-brain are lateral swellings, caused by 

 the outgrowing optic vesicles, but there is as yet no trace of the 

 lens. The auditory pits, EI, are a pair of shallow depressions, 

 with widely open mouths, at the sides of the hind-brain. 



