150 



EMBRYOLOGY. 



316. As a general fact, it should be further stated, that 

 the envelopes which protect the egg, and also the embryo, 

 are the more numerous and complicated as animals belong 

 to a higher class, and produce a smaller number of eggs. 

 This is particularly evident when contrasting the innumer- 

 able eggs of fishes, discharged almost without protection 



engaging 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, xx.') These walls, converging from 

 all sides upwards, rise gradually till they unite abo.ve 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 amnios. This cavity becomes 

 filled with a peculiar liquid, the amniotic water. 



315 c. Soon after the embryo has been 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, the allaniois, turning backwards 

 and upwards, so as completely to separate the two plates of the am- 

 nios, (Fig. 126, a,) and finally enclosing the whole embryo, with its 



Fig. 123. 



amnios, in another largo sac. The tubular part of this sac, which is 

 nearest the embryo, is at last transformed into the urinary bladder. 

 The heart (A) is already very large, with mniute arterial threads 



