232 GENERAL GROWTH OF THE EMBRYO. 



maxillary process (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. 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 

 develops a sharp flexure immediately behind the head, which is 

 thus bent forwards upon the posterior straight part of the body 

 (fig. 146). The amount of this flexure varies somewhat in differ- 

 ent 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 approximated. 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. 



Embryonic membranes and yolk-sack. 



The early stages in the development of the embryonic mem- 

 branes are nearly the same as in Aves ; but during the later 

 stages in the Placentalia the allantois enters into peculiar rela- 

 tions with the uterine walls, and the two, together with the 

 interposed portion of the subzonal membrane or false amnion, 

 give rise to a very characteristic Mammalian organ the 

 placenta into the structure of which it will be necessary to 

 enter at some length. The embryonic membranes vary so 

 considerably in the different forms that it will be advantageous 

 to commence with a description of their development in an ideal 

 case. 



