334 THE MAMMALIAN EMBRYO. [CHAP, 



rise both to the mouth and to the j)ituitary body. Be- 

 hind the mouth are three well marked pairs of visceral 

 arches. The first of these is the mandibular arch 

 (Fig. 108 md), which meets its fellow in the middle 

 line, and forms the posterior boundary of the mouth. 

 It sends forward on each side a superior maxillary pro- 

 cess {mx) which partially forms the anterior margin of 

 the mouth. Behind the mandibular arch are present a 

 well-developed hyoid (jiy) and a first branchial arch 

 (not shewn in Fig. 108). There are four clefts, as in 

 the chick, 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 fiexure appears, the 

 body also develops a sharp flexure immediately behind 

 the head, which is thus bent forwards upon the pos- 

 terior straight part of the body (Fig. 108). 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. 108), so 

 that the whole dorsal side assumes a convex curvature, 

 and the head and tail become closely approximated. In 

 most cases the embryo, on the development of the tail, 

 assumes a more or less definite spiral curvature (Fig. 

 108). With the more complete development of the 

 lower wall of the body the ventral flexure partially dis- 

 appears, 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. 



