274 VETERINARY STATE BOARD 



tern, the external sense-organs, as ear, eye, etc., and the mucous 

 membrane of the mouth and anus. 



What changes take place in the blood of pregnant animals? 



There is said to be an increase in the volume of the blood in the 

 body of a pregnant animal, but a decrease in its corpuscles and solid 

 constituents. Quite recently, investigators have proven the pres- 

 ence of a certain substance in the serum of pregnant animals which is 

 of value in diagnosing pregnancy. 



Name the envelopes surrounding the f CEtus and describe each. 



1. Chorion, the outer envelope, is a vast, membranous, closed 

 sac. It being moulded upon the uterine wall, resembles the uterus 

 in form, having a body and two cornua. The cornua are unequal in 

 size, the one in which the foetus lies being the largest. The external 

 surface of the chorion is studded with small, red tubercles, formed 

 by the placental villous tufts which join it to the internal surface of 

 the uterus. The internal surface is lined by the external layer of the 

 allantois, except at the umbilical cord, where there exists a kind of 

 conical inf undibulum, occupied by the umbilical vesicle. It is united 

 to the allantois by a layer of mucous connective tissue. 



The chorion not only plays a mechanical part in protecting the 

 foetus and supporting the placental blood-vessels, but it holds cer- 

 tain nutritive materials. 



2. The amnion, the second sac enveloping the foetus, is a thin, 

 transparent membrane. It floats freely in the interior of the chorion, 

 but is only united to it at one point through the medium of the 

 umbilical cord. It is ovoid in shape and contains the foetus which 

 is attached to its inner face by the vessels of the umbilical cord. 

 In the early embryonic stage, it closely envelops the embryo but, 

 later, it is distended with fluid (liquor amnii) and is thus separated 

 from the foetus. The smooth, internal face secretes the liquor amnii 

 which bathes the foetus and serves important functions. The exter- 

 nal face is closely adherent to the inner layer of the allantois. 



The amnion is made up of three layers : a very thin,, connective- 

 tissue membrane that adheres to the allantois; a membrane-proper 

 which is made up of connective tissue and some muscular fibres ; and 

 an epithelial layer which lines the latter. 



3. The allantois is a thin membrane which lines the inner face 

 of the chorion, and is reflected around the insertion of the umbilical 

 cord to be spread over the outer face of the amnion. By so doing, 

 it transforms the chorionic sac into a kind of serous cavity with the 

 amniotic sac enclosed within it. The allantoic cavity communicates 



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