^4j Dr Allen Thomson on the Vasctdar System 



In the horse, pig, cow and sheep, its growth is very rapid imme- 

 diately after its first appearance ; it fills the whole of the cornua 

 of the uterus, or of the compartment which each ovum occupies 

 (Fig. 32, z ^). It consists of two layers, the external being most 

 vascular, and is filled with a transparent and sometimes gelatin- 

 ous substance. 



In the cat and dog, again, the allantois envelopes the foetus 

 in its amnios, and the yolk, much in the same way as in the 

 eggs of birds (Fig. 33), leaving the yolk free on one side, how- 

 ever, during a considerable part of foetal life. 



In the Rod^ntia, the umbilical vesicle is so much developed, 

 that it appears to have taken the place of the allantois ; this lat^ 

 ter membrane is proportionally much less extended ; it re- 

 tains its vesicular form for a considerable time, and is enclosed 

 between the folds of the umbilical vesicle (Fig. 34). 



The outer layer of the allantois, from its forming a lining 

 to the envelope of the ovum, has been called Endochorion. 

 The umbilical vessels are ramified principally on this part of the 

 allantois, and being brought near into contact with the arte- 

 rial vessels distributed on the lining of the uterus, tlie respi- 

 ratory change of the blood in the umbilical arteries is thus for 

 some time performed. 



Very soon, however, after the allantois has expanded, so as 

 to fill the cavity of the uteius, or of the different compartments 

 occupied by the ova, and comes into contact with its parietes, 

 the Placenta, or principal respiratory organ of the foetal mam- 

 miferous animal begins to be formed. The structure of this part 

 may be most easily examined in the ova of ruminating animals, 

 as in the cow. In this animal, when the vascular or external 

 layer of the allantois lines the chorion, the extremities of the 

 umbilical vessels ramified on the allantois, leave that membrane 

 at particular parts, and join the chorion. This latter membrane 

 then becomes uneven at the places where these vessels join it ; 

 and as development proceeds, numerous little processes (Fig. 

 32, B) project from its external surface, upon which the capillary 

 vessels of the umbilical arteries are minutely ramified. These 

 processes gradually prolong themselves outwards, and carry 

 along with them the umbilical vessels, which gradually become 



