CHAP. XLIII.] 



ALLANTOIS. 



021 



Fig. 282. 



the late Mr. Dalrymple, who has made 

 some very successful injections of the 

 allantois of the chick. On the outer 

 surface that which is in immediate 

 contact with the membrane of the 

 shell the capillaries are exceedingly 

 abundant, and very minute. Mr. Dal- 

 rymple compares their arrangement to 

 that of the vessels in the air-cells of 

 the lungs of reptiles a resemblance 

 of great interest, when we consider 

 that in the bird this membrane per- 

 forms a most important part in aerat- 

 ing the blood ; indeed, it is through 

 the intervention of the allantois, that 

 all the respiratory changes taking place 

 in the chick are carried on. The air 

 passes through the pores in the shell 

 and membrane beneath, and thus is 

 brought into contact with the blood 

 ramifying in the vessels of the allan- 

 tois. 



The allantois, as was shown by Kei- 

 chert and others, is connected with 

 the efferent ducts of the Wolffian bo- 

 dies, and receives the secretion from 

 these glands. 



In the human subject, soon after 

 its formation, a dilatation is observed 

 in that part of the allantois nearest 

 the foetus. This is the rudiment 

 of the urinary bladder. Just at the 

 junction between the vesicular por- 

 tion and the straight tube which 

 passes from this point to the chorion, 

 a folding or constriction occurs. 

 This indicates the first formation of 



the UracJlUS, which has been errO- Diagrams to show the arrangement of the allan- 



neously considered to be situated tois ' an< 

 in that part between the vesi- 

 cle and the fcetus. This latter 



_ i* -i r cup-shaped cavities formed by the altered chorion. 



portion, however, SOOn becomes The allantois is of a very large size, and entirely 

 divided intO two tubes, One being Bounds the embryo. 



classes of animals: 



a. A portion of the wall of the uterus, ft. Chorion. 

 c. Allantois. d. Umbilical vesicle, e. Amnion. 



A. In ruminants. The cotyledons, spread out 

 over the internal surface of the uterus, n't into 



connected with each Wolffian 



In the ferae (cat, etc.), the placenta forms a 

 zone which surrounds the embryo like a ring. 

 There are no cotyledons. 

 4- -.V^o, n * n U,',,4.~1 c - In rodentia and in the human subject. The 



tabes are Ultimately placenta is limited to one particular part of the 

 chorion. The allantois is very small, and only 

 distinguished at a very early stage 



