86 



COMPARATIVE PHYSIOLOGY. 



fused growths from the junction of the decidua vera and sero- 

 tina is the decidua reflexa. 



The decidua serotiua and reflexa thus become the outermost 

 of all the coverings of the ovum. These and some other devel- 

 opments are figured below. It is to be remembered, however, 

 that they are highly diagrammatic, and represent a mixture of 

 inferences based, some of them, on actual observation and others 

 on analogy, etc. 



The figures will convey some information, though appear- 

 ances in all such cases must he interpreted cautiously for the 

 reasons already mentioned. 



During the first fourteen days villi appear over the whole 

 surface of the ovum. ; about this fact there is no doubt. At 

 the end of the first month of foetal life, a complete chorion 

 has been formed, owing, it would seem, to the growth of the 

 allantois (its mesoblast only) beneath the whole surface of the 

 subzonal membrane. From the chorionic surface vascular pro- 

 cesses clothed with epithelium project like the plush of velvet. 

 The allantois is compressed and devoid of a cavity, but abim- 

 dantly supplied with blood-vessels by the allantoic arteries and 

 veins, which of course terminate in capillaries in the villi. 

 Compare the whole series of figures. 



Fis. 88.— Human ova duriDg early stages of development. A aud 13, front and side 

 view of an ovum supposed to be about thirteen days old; e. embryonic area 

 (Qualn, after Heichert); C, ovum of four to five weeks, showing the general 

 structure of the ovum before formation of the placenta. Part of the wall of the 

 ovum is removed to show the embryo in position (after Allen Thomson). 



At this stage the condition of the chorion suggests the type 

 of the diffuse placenta which is normal for certain groups of 

 animals, as will presently be learned. 



The subsequent changes are much better understood, for 

 parts are iti general no longer microscopic but of considerable 

 size, and their real structure less readily obscured or obliterated. 



The amniotic cavity continues to enlarge by growth of the 

 walls of the amnion and is kept filled with a fluid ; the yelk-sac 



