IX.] THE BRAIN. 281 



Up to this period the walls of the somatic stalk 

 have remained thin and flaccid, almost membranous in 

 fact, the heart appearing to hang loosely out of the 

 body of the embryo. About this time however the 

 stalk, especially in front, rapidly narrows and its meso- 

 blast becomes thickened. In this way the heart and 

 the other thoracic viscera are enclosed by definite firm 

 chest walls, along the sides of which the ribs grow 

 forwards and in front of which the cartilaginous rudi- 

 ments of the sternum appear. 



The abdominal walls are also being formed, but not 

 to the same extent, and the stalk of the allantois still 

 passes out from the peritoneal cavity between the 

 somatic and the splanchnic stalks. 



In the brain one of the most marked features is the 

 growth of the cerebral hemispheres. The median division 

 between these has in front increased in depth, so that 

 the lateral ventricles are continued forwards as two 

 divergent horns, while backwards they are also con- 

 tinued as similar divergent horns separated from one 

 another by the vesicle of the third ventricle. 



We propose to treat more fully of the development of the 

 braiu in the second part of this work, the importance of the 

 mammalian brain rendering it undesirable to go too much into 

 the details of the brain of the bird. 



All the visceral clefts are closed by the seventh day. 

 It will be remembered that the inner part of the first 

 cleft persists as the Eustachian tube (p. 166). 



The structures which surround the mouth are be- 

 ginning to become avian in form, though the features 

 are as yet not very distinctly marked 



