46 



THE EARLY DEVELOPMENT OF MAMMALS. 



in the zona radiata. The variations offered in different mammals are so great that 

 a description less general than that given would hardly be applicable, 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 preco- 

 ciously, 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 ex- 

 pands to a considerable diameter before the 

 inner mass begins to spread out. The strik- 

 ing changes through which the inner mass 

 passes in the mole are illustrated in figure 12. 

 It forms at first a small globe, A. The 

 inner mass subsequently flattens out, becom- 

 ing 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 per- 

 haps in the mole, the outer of the two 

 FIG. 12. SECTIONS THROUGH THE INNER MASS OF layers is temporary only in existence. In 

 BLASTODERMIC VESICLES OF THE MOLE AT some rodents it acquires a very great de- 

 velopment and leads to the curious phe- 

 nomenon known as the inversion of the 

 germ-layers. The innermost of the layers, 

 Ent, grows at its edges, and its cells spread 



out gradually farther and farther under the subzonal layer until they extend com- 

 pletely around the vesicle 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 expand- 

 ing 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 

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



The point where the inner mass and the subzonal layers are connected with 

 one another marks the site of the future embryonic area. 



The blastodermic vesicle grows rapidly in size, partly by the multiplication of 

 its cells, partly by their becoming flattened out so as to cover a larger surface. 



THREE SUCCESSIVE STAGES. 



EC, Outer or subzonal layer, z, x, Zona pellucida. 

 i.m, Inner mass of cells. Ent, Entoderm. 

 (After W. Heape.) 



