DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 287 



scaly reptiles (snakes, lizards, and turtles), which have pecu- 

 liar membranes surrounding and protecting the embryo during 

 its growth. From one of these envelopes, the allantois (fig. 

 311, a), is derived their common name of allantoidian ver- 

 tebrata, in opposition to the naked reptiles and fishes, which 

 are called anallantoidian. 



473. The allantoidian vertebrata differ from each other 

 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. 310, x, x). 

 These walls, converging from all sides upwards, rise gradually 

 till they unite above the middle of the back (fig. 311). 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. 312, e) and the new membrane, 

 whose walls are called the amnios. This cavity becomes filled 

 with a peculiar liquid, the amniotic water. 



Fia:. 312. 



4/4. Soon after the embryo has been enclosed in the 

 amnios, a shallow pouch forms from the mucous layer below 

 the posterior extremity of the embryo, between the tail and 

 the vitelline mass. This pouch, at first a simple little sinus 

 (fig. 311, a), grows larger and larger, till it forms an extensive 

 sac, the allantois turning backwards and upwards, so as com- 

 pletely to separate the two plates of the amnios (fig. 312, a), 

 and finally enclosing the whole embryo, with its amnios, in 



