OUR EMBRYONIC DEVELOPMENT. 67 



comparison of the embryos of vertebrates, but also in 

 the comparison of their protective membranes. All 

 vertebrates of the three higher classes — reptiles, birds, 

 and mammals — are distinguished from the lower classes 

 by the possession of certain special foetal membranes, 

 the amnion and the serolemma. The embryo is 

 enclosed in these membranes, or bags, which are full 

 of water, and is thus protected from pressure or shock. 

 This provident arrangement probably arose during 

 the Permian period, when the oldest reptiles, the 

 proreptilia, the common ancestors of all the amniotes 

 (animals with an amnion), completely adapted them- 

 selves to a life on land. Their direct ancestors, the 

 amphibia, and the fishes are devoid of these foetal 

 membranes ; they would have been superfluous to 

 these inhabitants of the water. With the inheritance 

 of these protective coverings are closely connected 

 two other changes in the amniotes : firstly, the entire 

 disappearance of the gills (while the gill arches and 

 clefts continue to be inherited as " rudimentary 

 organs") ; secondly, the construction of the allantois. 

 This vesicular bag, filled with water, grows out of the 

 hind-gut in the embryo of all the amniotes, and is 

 nothing else than an enlargement of the bladder of 

 their amphibious ancestors. From its innermost and 

 inferior section is formed subsequently the permanent 

 bladder of the amniotes, while the larger outer part 

 shrivels up. Usually this has an important part to 

 play for a long time as the respiratory organ of the 

 embryo, a number of large blood vessels spreading 

 out over its inner surface. The formation of the 

 membranes, the amnion and the serolemma, and 

 of the allantois, is just the same, and is effected 

 by the same complicated process of growth, in 



