492 THE HUMAN EMBRYO. 



The olfactory pits, OF, are a pair of shallow depressions, on 

 the under surface of the extreme anterior end of the head, 

 above the mouth. The lens of the eye has not yet commenced 

 to form ; and the auditory vesicles, El, are now a pair of closed 

 sacs, embedded in the mesoblast at the sides of the hind-brain. 



The gill-cleft region is a triangular patch on each side of 

 the neck, with the apex directed backwards, and is bounded both 

 dorsally and ventrally by shallow grooves. The hyomandibular 

 cleft, HM, and the first, second, and third branchial clefts are all 

 represented by pharyngeal pouches, and by external grooves 

 corresponding to them ; but none of the clefts are actually 

 perforated. 



The stomatodasal pit is much deeper, and more clearly defined 

 than before, owing to uprising of its lips : it now opens into 

 the fore-gut. 



The heart, RA, RV, RT, is larger than before, and its several 

 divisions are more clearly marked off from one another by 

 constrictions. The whole heart has shifted backwards to a 

 certain extent, the greater part of it now lying behind the gill- 

 cleft region. 



The somites, or protovertebrae, are more distinct than before. 

 A pair of longitudinal ridges which run along the sides of the 

 body, ventral to the somites, are spoken of as the Wolffian 

 ridges. Each of these ridges is more prominent at two places, 

 opposite the posterior end of the heart, and opposite the allantoic 

 stalk respectively. These more prominent parts of the continu- 

 ous Wolffian ridges are the rudiments of the arms and legs. 



4. The Fourth Week. 



The fourth week is marked by a great increase in the size of 

 the embryo, growth being relatively more active at this than at 

 any other period. 



In the early part of the fourth week (Figs. 199-201), the 

 flexure of both head and body is very strongly marked, the 

 embryo being rolled up on itself so that the head and tail touch 

 or even overlap, and the outline of the entire embryo being 

 approximately circular. The several parts of the head are more 

 conspicuous than before ; the visceral clefts and arches are more 

 clearly defined ; the nose and ear are more prominent ; the heart 

 is very large ; the Wolffiau ridges are still present as con- 



