722 



differentiated; in the anterior two-thirds only of the pigment 

 layer is pigmentation present; there is still a lumen in the lens; 

 and a very minute lumen can still be traced in the optic stalk. 

 The internal ear much resembles that in the previous embryo, but 

 here, though the cochlea is still moderately straight at its anterior 

 end, the first indications of coiling have commenced. 



Heart and vessels. — At birth the permanent circulation becomes 

 established, and even already in this specimen the ductus 

 arteriosus is no longer patent, and the umbilical vein is obliterated. 

 The foramen ovale is closed. 



Lungs. — The lungs at birth exhibit a very interesting condition. 

 The bronchi are as yet but very imperfectly formed, and the 

 ramifications but few, and as the lungs have at this early stage 

 of development to be functional, the bronchi have become dilated 

 into very large irregular air-spaces to increase the surface of the 

 mucous membrane. The trachea and the upper part of the two 

 main bronchi are supported by imperfect cartilaginous rings. 

 The air-sacs open either directly into the main bronchi or into 

 the secondary bronchi. Connected with the bronchi are numerous 

 comparatively small irregular tubular ramifications, which 

 probably represent the further development of the bronchial 

 system, and which as yet are probably not functional. 



Alimentary canal. —The alimentary canal is now well formed, 

 and in the stomach and small intestine are coagula of milk. The 

 small intestine is of large size to near its lower end. It is 

 arranged in about four very irregular coils. Its lower 6th is 

 comparatively small. The cjecum is still small, though con- 

 siderably longer than in the earlier embryos. The colon is 

 relatively small and simple. Judging from the structui-e of the 

 alimentary canal it seems probable that all the milk ingested 

 becomes absorbed before reaching the colon, and that the cascum 

 and colon are not functional till a much later j^eriod. 



Urinogenital system. — The mesonephros is still of very large 

 size, and the tubules very much convoluted. At this stage, and 

 probably for some time afterwards, the entire excretory function 



