334 THE MAMMALIAN EMBRYO. [CHAP. 



rise both to the mouth and to the pituitary body. Be- 

 hind the mouth are three well marked pairs of visceral 

 arches. The first of these is the mandibular arch 

 (Fig. 108 md) y 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 (hy) 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 flexure 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. 



