THE ONTOGENESIS OF VERTEBRATES 73 



cialized organ, is termed detiduate, the former indeciduate. 

 In ungulates and in many of the edentates the placenta is in- 

 deciduate, in most others it is deciduate. 



When the fertilized egg of a placental mammal first enters 

 the uterus it does not at once become fixed, and development 

 proceeds for some time before there is any attempt at the for- 

 mation of a placenta. Meanwhile the egg passes through a 

 series of typical cleavage stages and attains the condition of a 

 hollow sphere, similar to the blastula of more typical onto- 

 genesis. This, however, is not a blastula, but the blastodermic 

 vesicle, upon one side of which there develops an embryonal 

 area similar to that of the bird, that is, spread out in the form 

 of a flattened disc, and not cylindrical as in the case of other 

 yolkless eggs. This apparently useless circumlocution can be 

 understood only on the ground, supported also by the early 

 development in the marsupials and monotremes, that mammals 

 have been derived from ancestors having large, yolk-filled 

 eggs and that the secondary reduction of this substance has 

 been too recent to effect a corresponding modification in the 

 course of development. Adhesion to the walls of the uterus 

 occurs through the formation of chorionic villi over the sur- 

 face of the blastodermic vesicle, in which the form of placen- 

 tation characteristic of the species soon becomes manifest. 



The later developmental history of vertebrates subsequent 

 to the formation of the germ layers and the establishment of 

 the anlagen of the various systems, belongs to that division 

 of the subject known as organogeny, or the development of 

 the various organs, and cannot be followed ' further in this 

 place; it receives a fuller treatment, however, in the ensuing 

 chapters, where the systems are considered separately and 

 where embryological facts are made use of in so far as they 

 are needed to explain the history of the several organs. Most 

 of the systems arise from a single germ layer, often, indeed, 

 from a definite restricted locality in one of them, the anlage of 

 which appears at an early period, and there is thus a time at 

 which an organ, however complex and difficult to understand 

 as it exists in the adult, is exceedingly simple. This primitive 



