60 THE EARLY DEVELOPMENT OF MAMMALS. 



envelope itself, which, therefore, appaars, when the ovum is removed from the 

 uterus, to be studded over with fine threads resembling villi. The gelatinous 

 envelope has been termed by Hensen the pro-chorion. The thread-like pro- 

 jections seen in the dog were taken by Bischoff for true villi, and they have some- 

 times been referred to as the pro-chorionic villi. The term pro-chorion has been 

 applied to other structures, as, for instance, to the subzonal layer of the blasto- 

 dermic vesicle. The student needs to be warned against confusing the term 

 pro-chorion in its various applications. 



The Blastodermic Vesicle. 



The blastodermic vesicle always consists at first of the subzonal layer and 

 an inner cell mass attached at one point to the subzonal layer, and has a cavity 

 between the inner mass and the subzonal layer; the vesicle itself is always 

 enclosed in the zona radiata. The variations offered in different mammals are 

 so great that a description less general than that given would hardly be appli- 

 cable, even to the placental mammals. 



The next step in development is the production of a complete second layer 

 out of the cells of the inner mass. This layer extends completely around the 

 vesicle and lies close against the subzonal layer, and encloses the main cavity 

 of the vesicle. The way in which this inner vesicular layer is developed varies 

 greatly. In the hedgehog it appears very precociously, while the blastodermic 

 vesicle is very small, and afterward it expands rapidly, while the vesicle as a 

 whole is growing. In the rabbit and in the mole it is formed much later, and the 

 one-layered vesicle expands to a considerable diameter before the inner mass 

 begins to spread out. The striking changes through which the inner mass passes 

 in the mole are illustrated in Fig. 17. It forms at first a small globe, A. The 

 inner mass subsequently flattens out, becoming lens-shaped, thinner, and larger 

 in area, B. It continues spreading laterally and separates into three layers. 

 The two outer layers enter into the formation of the true ectoderm, C. In the 

 rabbit, and perhaps in the mole, the outermost layer is temporary only in exist- 

 ence. In some rodents it acquires a very great development and leads to the 

 curious phenomenon known as the inversion of the germ-layers. The innermost 

 of the layers grows at its edges, and its cells spread out gradually further and 

 further under the subzonal layer until they extend completely around the vesi- 

 cle and form, by meeting at the opposite pole of the ovum, a closed vesicle. 

 Very similar is the process in the rabbit. The cells at the expanding edge of the 

 inner layer are found to spread rapidly, so that during the expansion they are 

 more or less widely separated from one another. But they continue their expan- 

 sion and multiplication until they form a complete inner epithelial layer. 



