ORGANIC EVOLUTION 



215 



mammals, and it has many features of unique interest and 

 significance, but we can here consider only a few of its 

 more general aspects. 



We have already seen in the salamander that a dilata- 

 tion of the oviduct near its lower end (the uterus) serves for 

 the temporary storage of the ripe eggs, just before their 

 extrusion. In the higher mammals the two oviducts 

 (called also Fallopian tubes) become confluent at this 

 portion of their length into a single uterus, in which 

 the eggs on leaving the ovaries find lodgment. Being 

 fertilized internally they remain here, and undergo seg- 

 mentation and other early developmental changes while 

 lying against the uterine wall. Almost as soon as the 

 primary germ layers are established the ectoderm of the 

 ventral wall rises up about the embryo in a circular fold all 

 about its body and over its back; the edges of the fold 

 come together and fuse and enclose the embryo (or foetus) 

 under a double canopy of thin membrane called the 

 amnion (fig. 1376). Almost simultaneously a food 

 absorbing organ called the allantois develops for at- 

 tachment of the embryo to the wall of the uterus. 

 This springs from the endoderm near the posterior end 

 of the archenteron. It grows out as a hollow membranous 

 fold posteriorly and then dorsally between the wider folds of 

 the amnion; there is developed within the allantois a 

 complete set of embryonic blood vessels, the principal ones 

 being an allantoic artery that springs from the great dorsal 

 aorta, and an allantoic vein that returns the blood to the 

 post cava. The allantois and the outer layer of the amnion 

 become fused together, and attached to the uterine wall in a 

 series of minute interlocking processes (villi), the whole 

 complex attachment layer being known as the placenta. 

 The processes on the wall of the uterus become permeated 

 by a dense network of capillaries developed from the blood 



