*M the Fcetus of Vertebrated Animals, J8^v 



larger and more numerous *. At tlie same time, the projections 

 situated on the inner membrane of ihe uterus, corresponding 

 in position and form with those on the chorion of the ovum, be- 

 come enlarged ; into these processes of the uterus the projecting 

 parts of the chorion are gradually inserted. These placental 

 processes of the mother are also very vascular, so that the umbi- 

 lical arteries of the foetus carrying venous blood, are brought 

 into contact with those carrying the arterial blood of the uterus, 

 by the influence of which the necessary change or arterialization 

 seems to be effected f . 



After a little more than a sixth of the time of uterine gesta- 

 tion has elapsed, at which period the foetus of the mammi- 

 ferous animal corresponds in its structure to the chick in ovo 

 on the third and fourth days, rudiments of a branchial appara- 

 tus, analogous to those already alluded to in the higher reptiles 

 and in birds, are to be found. We are indebted chiefly to 

 Rathke, Baer, and Burdach for the discovery and elucidation 

 of these interesting facts. The observations of these authors 

 have principally been made on the erabryoes of the cow, pig, 

 sheep, dog, rabbit, and of the human species, and the appear- 

 ances they have observed in all of these animals have been so 

 similar, as to warrant the conclusion, that they are common to 

 all or most of the raammiferous families. The general features 

 in the structure of the neck and pharynx, which assimilate the 

 embryo of the mammiferous animal to that of the aquatic ani- 

 mal in tlie early stages of their development, are the same 

 as those already mentioned in birds. They consist in the 

 shortness and thickness of the neck, the width of the pharyn- 

 geal portion of the intestine, the penetration of its sides by 

 clefts, and the subdivision of the aorta into vessels corresponding 

 in number and distribution with the primitive branchial arteries. 



Four openings on each side of the oesophagus have been 

 observed in the embryo of the dog, between three and four 



• See a paper by Sir E. Home, PhiL Trans, vol. cxii. ; and Burdach's 

 Physiol., B. ii. S. 634. 



•f For an account of the varieties of the form and nature of the placenta in 

 different classes of animals ; see another paper by Sir E. Home in the PhiL 

 Trans — Carus' Comparative Anatomy, 2d vol— Jeffray de Placenta, &c 



JAXUAEY — MARCH 1831. S 



