THE ONTOGENESIS OF VERTEBRATES 71 



ishes, so that by the time the animal assumes a free life the 

 yolk-sac has nearly or wholly disappeared. 



The embryos of the higher vertebrates differ from those 

 of the lower in one very conspicuous feature, and that is, in 

 the possession of fetal membranes, external to the embryo and 

 designed in part for protection and in part for the obtaining 

 of nourishment. The two membranes of the most extensive 

 occurrence are the amnion and the allantois, which are present 

 in reptiles, birds and mammals and absent in fishes and am- 

 phibians, a difference which is expressed in the two terms 

 Amniota and Anamnia (with and without amnion), applied 

 respectively to the two divisions in question. 



The amnion appears to be solely for the protection of the 

 embryo. It is a thin transparent membrane, composed of parie- 

 tal mesoderm and ectoderm, and is formed by the growth of 

 folds about the embryo. It invests the latter on all sides and 

 forms about it an enclosed space, the amniotic cavity, in which 

 the embryo lies, immersed in a colorless amniotic fluid, of 

 about the same specific gravity as the embryo itself. The 

 allantois is in the form of an empty sac, composed of two 

 layers, visceral mesoderm and endoderm, and develops from 

 the umbilical region of the embryo. In reptiles and birds it 

 pushes its folded edges between yolk-sac and amnion on the 

 inner, and the shell on the outer, side, and thus comes to com- 

 pletely invest the former and line the latter with a double mem- 

 brane. In this there develop two large allantoic (umbilical) 

 arteries and two allantoic veins, and the organ thus serves as 

 an excellent respiratory organ, affecting the interchange of 

 gases through the porous shell. In placental mammals the 

 egg-shell is replaced by a membranous chorion, and the allan- 

 tois effects a close union with this, either involving the entire 

 surface or more generally a restricted area, and this surface, 

 entering into a more or less intimate relationship with the 

 mucous membrane of the maternal uterus, forms the essential 

 organ of nutrition, the placenta. That portion of the chorion 

 which is involved in the formation of a placenta is covered by 

 branching processes, the chorionic villi, forming a surface 



