134 COMPARATIVE ANATOMY. 



that of the preceding class marked. A distinct metamor- 

 phosis seen in young of Rana (tadpole). The features of this 

 stage, viz,, presence of gills and tail, are persistent in many 

 batrachians, as Axolotl. 



(2) After the formation of the neural canal, and the enclo- 

 sure of the vitelline sac by the outer germinal layer is com- 

 plete, a fold of the same layer arises up around the embryo 

 (amniotie fold) which, uniting above, forms a distinct cavity 

 (amnion), in which is contained a clear fluid. Soon after the 

 development of this cavity, a pouch-like depression of the 

 internal layer of germinal membrane is seen to one side of 

 the umbilical sac at the posterior and inferior part of the 

 visceral cavity. It extends outward and upward parallel with 

 the curve of the amniotie fold, and is called the allantois.* 



Reptilia and Aves.. Here the ova are developed inde- 

 pendent of maternal connection. The allantois is of propor- 

 tionately large size, covering the upper portion of the embryo 

 and lying between it and the porous shell. It is at once a 

 respiratory and excretory organ to the embryo. 



Mammalia. Here the ova are borne by the mother either 

 during a portion or the whole of the period of development; 

 the allantois presenting two varieties of structure. The first 

 of these is of comparatively small size, and free, as in impla- 

 cental mammals. In the second the organ, often completely 

 surrounding the egg, early becomes united with the chorion, 

 as in placenta! mammals. In the implacental, the chorion is 

 either smpoth (Monotremata) or transversely wrinkled (Mar- 

 supialia), and has little or no means of retention to the uterine 

 wall. In the placental, on the other hand, the chorion is 

 shaggy and has partial or entire connection with the analo- 

 gous formation of the mucous membrane of the uterus. 



The placenta is the organ of respiration in mammals, as 



* Prof. Huxley gives a different account of the formation of this organ : 

 "The allantois is developed much later than the amnion, neither from the 

 serous nor from the mucous layers of the germ, but from that intermediate 

 stratum whence the bones, muscles, and vessels are evolved. It arises as a 

 solid mass from the under part of the body of the embryo behind the 

 primitive intestinal cavity." 



