192 



GENERAL GROWTH OF THE EMBRYO. 



the posterior boundary of the mouth. It sends forward on each side 

 a superior maxillary process (mx) which partially forms the anterior 

 margin of the mouth. Behind the mandibular arch are present a 

 well-developed hyoid (hi/) and a first branchial arch (not shewn in 

 fig. 146). There are four clefts, as in other Amniota, but the fourth 

 is not bounded behind by a definite arch. Only the first of these 

 clefts persists as the tympanic cavity and Eustachian tube. 



At the time when the cranial flexure appears, the body also 



t/nt 



Fir;. 140. ADVANCED EMBRYO OF A BABBIT (ABOUT TWELVE DATs) 1 . 

 ml), mid-brain; ill. thalamencephalon ; ce. cerebral hemisphere; op. eye; iv.v. 

 fourth ventricle; mx. maxillary process; md. mnndibular arch; hi/, hyoid arch; //. 

 fore-limb; /;/. hind-limb; um. umbilical stalk. 



develops a sharp flexure immediately behind the head, which is thus 

 bent forwards upon the posterior straight part of the body (fig. 140). 

 The amount of this flexure varies somewhat in different forms. It is 

 very marked in the dog (Bischoff). At a later period, and in some 

 species even before the stage figured, the tail end of the body also 

 becomes bent (fig. 146), so that the whole dorsal side assumes a 

 convex curvature, and the head and tail become closely approxi- 

 mated. In most cases the embryo, on the development of the tail, 

 assumes a more or less definite spiral curvature (fig. 146); which 

 however never becomes nearly so marked a feature as it commonly 

 is in Lacertilia and Ophidia. With the more complete development 

 of the lower wall of the body the ventral flexure partially disappears, 

 but remains more or less persistent till near the close of intra-uterine 

 life. The limbs are formed as simple buds in the same manner as in 

 Birds. The buds of the hind-limbs are directed somewhat forwards, 

 and those of the fore-limb backwards. 



1 This figure was drawn for me by my pupil, Mr Wheldon. 



