FCETAL MEMBRANES 437 



In certain Amphibians which have no free larval existence, 

 interesting modifications occur for nourishing the young until the 

 larval stage is passed. Thus in the Alpine Salamander (Salaman- 

 dra atra), a large number of ova (40 60) pass into each oviduct, 

 just as in the allied S. maculosa, in which the young are born as 

 gilled larvae. Were this the case in S. atra, the young would be 

 carried away in the mountain streams and destroyed, and a 

 curious adaptive modification has arisen in this form, in 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 complete metamorphosis 

 has taken place. The other eggs break down and form a food- 

 mass for the survivors after their own yolk is used up. Degenerative 

 changes, moreover, take place in the epithelium of the oviduct, 

 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. The long, feather-like external gills are 

 closely applied to the uterine mucous membrane, and so effect an 

 interchange of gases in respiration. After the young is born, the 

 uterine epithelium becomes regenerated, and thus a process occurs 

 which somewhat resembles that of the formation of a decidua in 

 placental Mammals. 



II. AMNIOTA. 



In all the Amniota, as already mentioned (p. 9), foetal mem- 

 branes, known as the amnion and allantois are developed, and the 

 latter, or primary urinary bladder, represented only in rudiment 

 in the Amphibia, is of great importance in connection with respira- 

 tion, excretion, and (in the higher Mammals) nutrition in the 

 embryo. 



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

 development, the amnion * consists primarily of two layers ; an 

 inner, the amnion proper, and an outer or false amnion. The 

 latter comes to lie close to the vitelline membrane, and forms the 

 so-called serosa, or serous membrane. As the allantois grows, it 

 extends into the space continuous 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 each pole of the embryo ; the latter of these is 

 the more important. Both foetal and maternal parts of the 



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 ectoderm and called the 

 proamnion : this is afterwards replaced by the true amnion. 



