204 THE SIXTH DAY. [CHAP. 



chorion, (or outer primitive amniotic fold,) is by the comple- 

 tion of the cleavage of the mesoblast and the invagination 

 of the yolk-sac, entirely separated from the embryo. The 

 cavity of the allantois by means of its stalk passing through 

 the umbilicus is of course continuous with the cloaca. 



6. In the embryo itself a few general points deserve 

 notice. 



By the sixth or seventh day, the flexure of the body has 

 become less marked, so that the head does not lie so near 

 to the tail as on the previous days; at the same time a 

 more distinct neck makes its appearance. 



Though the head is still disproportionately large, its 

 growth ceases to be greater than that of the body. 



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 mesoblast becomes thickened. 

 In this way the heart and the other thoracic viscera are en- 

 closed by definite firm chest walls, along the sides of which 

 the ribs grow forwards and in front of which the cartilaginous 

 rudiments 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 continued 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 brain in a later 

 part of this work, the importance of the mammalian brain rendering it un- 

 desirable to go too much into the details of the brain of the bird. 



All the visceral clefts, with the exception of the first, are 

 closed by the seventh day: this one however still remains 

 open, communicating with the mouth by the Eustachian tube 

 and with the exterior by the aperture of the external auditory 



