THE FCETUS IN UTERO. 85 



spermatozoon of the rabbit and horse do not appear to differ 

 materially from each other, we are here brought face to face 

 with one of the mysteries of creation, a mystery which we can 

 no more explain than we can explain why in the adult being 

 one cell or particle of living matter should always abstract from 

 the blood and elaborate into tissue the materials of bone, another 

 those of muscle, and another of nervous tissue. 



MEMBRANES OF THE FCETUS. 



Our present purpose does not demand that we should trace 

 the development of the ovum in all its stages into the foetus. I 

 hasten, therefore, to note the connection of the ftetus with the 

 mother, and how it is nourished in the latter period of gesta- 

 tion. 



The foetus floats in one water bag (Amnion) enclosed in 

 another (the AUantois), which among other functions fulfil that 

 of protecting the young animal from being injured by the move- 

 ments of the abdominal organs, or by external objects coming 

 in contact with the abdomen, and that of steadily dilating the 

 external generative passages by a soft, equable and yielding 

 pressure, preparatory to the expulsion of the foetus. The 

 Amnion likewise receives any dejections in case the bowels act 

 before birth, while the AUantois is the receptacle for the urine 

 which is conveyed from the anterior extremity of the bladder 

 through a special channel (uracus) in the navel string. Out- 

 side the AUantois and lining the womb is the vascular coat (the 

 chorion), whose functions are the most pertinent to our present 

 purpose. The blood of the foetus is conveyed to this membrane 

 by the two umbilical arteries, branches of the internal iliacs, 

 and after breaking up into capillaries in its substance is returned 

 by the umbilical vein. Branches are given off from these vessels 

 for the nourishment of the three membranes, but the blood is 

 mainly distributed on the villous process of the chorion to 

 absorb the nutrient matters from the blood of the mother. 



NUTRITION OF THE FCETUS. 



The inner surface of the mucous membrane of the womb, even 

 in the unimpregnated state, is perforated by numerous orifices 

 leading into two kinds of uterine follicles, one consisting in 

 simple depressions terminating in blind ends, the other consist- 



