DEVELOPMENT OF THE YOUNG WITHIN THE* EGG. 119 



sumes its final position at the most projecting point of the 

 head. The yolk is suspended from the belly, in the form of 

 a large bladder, but it daily diminishes in size, until it is 

 at length completely taken into the animal. The duration 

 of these metamorphoses varies extremely in different fishes ; 

 some accomplish it in the course of a few days, while in 

 others months are required. 



315 a. In frogs and all the naked reptiles, the development is very 

 similar to that of fishes. It is somewhat different in the scaly rep- 

 tiles (snakes, lizards and turtles), which have peculiar membranes 

 surrounding and protecting the embryo during its growth. From one 

 of these envelops, the allanto'is (Fig. 125, a,) is derived their common 

 name of Allan to'idian Vertebrates, in opposition to the naked reptiles and 

 fishes, which are called Anallanto'idian. 



315 6. The Allantoidian Vertebrates differ among themselves in several 

 essential peculiarities. Among Birds, as well as in the scaly reptiles, we 

 find at a certain epoch, when the embryo is already disengaging itself from 

 the yolk, a fold rising around the body from the upper layer of the germ, 

 so as to present, in a longitudinal section, two prominent walls (Fig. 124, 



Fig. 124. Fig. 125. 



xx). These walls, converging from all sides upwards, rise gradually 

 till they unite above the middle of the back (Fig. 125). When the 

 junction is effected, which in the hen's egg takes place in the course 

 of the fourth day, a cavity is formed between the back of the embryo 

 (Fig. 126, e) and the new membrane, whose walls are called the am- 

 nios. This cavity becomes filled with a peculiar liquid, the amniotic 

 water. 



315 c. Soon after the embryo becomes enclosed in the amnios, a 

 shallow pouch forms from the mucous layer below the posterior ex- 

 tremity of the embryo, between the tail and the vitelline mass. This 

 pouch, at first a simple little sinus (Fig. 125, a), grows larger and larger, 

 till it forms an extensive sac, bending backwards and upwards, so as to 



