MAMMALIA. 



231 



that the head becomes quite free, and a considerable portion of 

 the throat, ending blindly in front, becomes established. In the 

 course of this folding the, at first widely separated, halves of the 

 heart are brought together, coalesce on the ventral side of the 

 throat, and so give rise to a median undivided heart. The fold 

 at the tail end of the embryo progresses considerably, and dur- 

 ing its advance the allantois is formed in the same way as in 

 Birds. The somites increase in number to about twelve. The 

 amniotic folds nearly meet above the embryo. 



The later stages in the development proceed in the main in 

 the same manner as in the Bird. The cranial flexure soon be- 

 comes very marked, the mid-brain forming the end of the long 

 axis of the embryo (fig. 146). The sense organs have the usual 

 development. Under the fore-brain appears an epiblastic invo- 

 lution giving rise both to the mouth and to the pituitary body. 

 Behind the mouth are three well-marked pairs of visceral arches. 

 The first of these is the mandibular arch (fig. 146, 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 



nib 



FIG. 146. ADVANCED EMBRYO OF A RABBIT (ABOUT TWELVE DAYS)'. 



nib. mid-brain; th, thalamencephalon ; cc. cerebral hemisphere; of. eye; iv.v. 

 fourth ventricle; mx. maxillary process ; md. mandibular arch ; hy. hyoid arch;y7. 

 fore-limb; hi. hind-limb; nm. umbilical stalk. 



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



