626 VETERINARY PHYSIOLOGY 



floats during the latter stages of its development, and which acts 

 as a most efficient protection against external violence. The 

 source of this fluid has been much debated. In birds it is 

 certainly of foetal origin. In mammals it has been contended 

 that it is derived from the maternal circulation. But, since in 

 herbivora it resembles urine more than a blood transudate and 

 since the urethra of the foetus opens into the amniotic sac, it is 

 probably chiefly derived from the foetal kidneys. In rabbits, 

 when the foetus is killed in utero, no fluid is formed in the sac, 

 although the maternal part of the placenta persists. 



A very significant fact is that in herivora it contains a sugar, 

 Isevulose, which is present in the foetal blood but not in the 

 maternal blood. 



II. Attachment to the Mother. 



(1) By the action of its ectoderm cells the ovum burrows 

 its way into the mucous membrane of the uterus which 

 is hypertrophied and very vascular. These ectoderm cells 

 grow outwards as syncytial masses of protoplasm forming the 

 trophoblast layer. The burrowing action may be due to the 

 development of some powerful proteolytic enzyme, although 

 definite proof of its existence is not forthcoming. 



Certainly, in some way the maternal tissues are killed and 

 digested. When a maternal blood-vessel is opened into, the 

 blood is hsemolysed, thus probably rendering the iron of the 

 hsemoglobin available for absorption by the embryo. 



At this stage of development the embryo is a parasite upon 

 the mother living upon her substance. 



The important part played by the corpus luteum in de- 

 termining the implantation of the ovum has been discussed on 

 p. 608. 



(2) Later, the mesoblast of the embryo extends out in a 

 number of finger-like processes into the trophoblast layer, and 

 soon afterwards blood - vessels shoot into these, and the 

 chorionic villi are formed. These are at first covered by a 

 definite layer of cubical cells, the layer of Langhans with 

 outside it a syncytial trophoblast layer of protoplasm. 

 Later the layer of Langhans disappears and the syncytium 

 becomes extremely thin (fig. 242). 



