248 COMPARATIVE ZOOLOGY OF CHORDATES 



extruded, and the egg of the Placental mammals contains no 

 yolk. Nevertheless, in both the last-mentioned groups a 

 yolk-sac is present although it contains no yolk. 



In several groups of vertebrates, the yolk-sac may come to 

 bear interesting relations to the wall of the oviduct, with which 

 it is in contact if the egg is not laid but undergoes development 

 within the body of the mother. The blood-vessels of the 

 yolk-sac are in these cases able to absorb substances from the 

 circulation of the mother (by diffusion), and such an organ of 

 physiological communication between mother and embryo is 

 a placenta. It is necessary to specify the organ which forms 

 the placenta, and a placenta derived from the yolk-sac is called 

 an omphaloidean placenta, to distinguish it from the allantoic 

 placenta which is formed by the allantois. 



An omphaloidean placenta is present in the dogfish 

 Mustelus, and in the reptile Chalcides ; it is the chief nutritive 

 organ in the embryonic development of the Marsupials (in 

 fact, it is the only form of placenta in all except Perameles 

 which in addition has an allantoic placenta), and in the 

 " Placental " mammals it arises early and disappears later. 



As development proceeds, and the quantity of yolk is 

 reduced, the size of the yolk-sac decreases and finally it is 

 withdrawn into the body through the umbilical stalk. 



The Allantois. — The allantois occurs in reptiles, birds, and 

 mammals, and attains its greatest development in the latter. 

 It develops as an outgrowth from the hind part of the gut, and 

 is an endodermal sac covered with mesoderm in which blood- 

 vessels run. In amphibia it is represented by the (allantoic) 

 bladder. In reptiles, birds, and Monotremes, the allantois 

 functions as a respiratory and excretory organ, for which it 

 is well fitted since the excretory ducts open into its base, and 

 its distal portion is spread out close beneath the (porous) shell. 

 In the reptile Chalcides, the Marsupial Perameles, and the 

 Placental mammals, the allantois enters into relations with the 

 wall of the oviduct (or uterus) and forms the allantoic placenta. 

 Its function is then nutritive as well as respiratory and 

 excretory. It is easy to see how this may have occurred in 

 evolution by the retention of the egg within the oviduct and 



