FCETAL MEMBRANES 337 



which only one embryo (that nearest the cloaca) in each oviduct 

 undergoes complete development, remaining within the body of 

 the parent until the gills are lost and metamorphosis has taken 

 place. The other eggs break down and form a food-mass for 

 the survivors after their own yolk has become used up. Degene- 

 rative changes, moreover, take place in the epithelium of the ovi- 

 duct, and masses of red blood-corpuscles pass into the lumen of the 

 latter, undergo degeneration, and become mixed with the broken- 

 down yolk-masses, the resulting broth being swallowed by the 

 surviving young. After the birth of the latter, the uterine epithe- 

 lium becomes regenerated ; and thus a process occurs which some- 

 what resembles that of the formation of a decidua in placental 

 Mammals (p. 340). 



II. AMNIOTA. 



In all the Anmiota, as already mentioned (pp. 9 and 302), foetal 

 membranes, known as the amnion and allantois are developed, 

 the latter, or primary urinary bladder, represented only in rudi- 

 ment in the Amphibia (p. 259), being of great importance in con- 

 nection with respiration, secretion, and (in the higher Mammals) 

 nutrition of the embryo. 



A glance at Fig. 8 will show that, owing to its mode of develop- 

 ment, the amnion 1 consists primarily of two layers ; an inner, the 

 amnion proper, and an outer or false amnion. The latter lies close 

 to the vitelline membrane, and forms the so-called serosa, or serous 

 membrane. As the allantois grows it extends into the space con- 

 tinuous with the ccelome between the true and false amnion, and 

 may entirely surround the embryo. 



Amongst Reptiles, the eggs of the viviparous Lizard, Seps chal- 

 cides, are relatively poor in yolk, and this is compensated for by 

 the yolk-sac and allantois coming into close relation with the 

 walls of the oviduct, thus forming an umbilical and an allantoic 

 placenta, one at either pole of the embryo ; the latter of these is 

 the more important. Both foetal and maternal parts of the pla- 

 centae become extremely vascular, and thus the necessary inter- 

 change of materials can take place between the blood of the em- 

 bryo and mother. In Trachydosaurus and Cyclodus, as well as in 

 the Chelonia, a kind of umbilical placenta is apparently also 

 formed. . 



The fact that a vascular yolk-sac (often known as the umbilical 

 vesicle) is present in placental Mammals, indicates that they are 

 descended from forms in which, like the Sauropsida, the eggs were 

 rich in yolk, and which were viviparous. This condition is 



1 As the head enlarges and sinks downwards, it is at first surrounded by a 

 modification of the head fold (p. 9) consisting entirely of epiblast and called the 

 pro-amnion : this is afterwards replaced by the amnion. 



Z 



