04 



THE EARLY DEVELOPMENT OF MAMMALS. 



of rounded cells heavily laden with yolk-granules, and containing conspicuously 

 large nuclei. These large nuclei differ by their size and minute structure very 

 much from the other nuclei in the embryo. The corresponding nuclei in higher 

 animals are sometimes called parablast nuclei. Outside of the entoderm comes the 

 second portion of the yolk-sac, the splanchnic leaf of the mesoderm. If we 

 imagine the amount of yolk to be gradually increased, so that it would appear 

 more distinct from the embryo proper, we should then apply to it the term extra- 

 embryonic. The yolk-sac of the higher forms differs from that of the lower forms 

 only by its size, as is illustrated by figure 29, which represents a diagrammatic 

 transverse section of an early stage of the chick, before the formation of the 

 amnion has begun. The essential relations may be seen by comparing figures 29, 



Ent 



FIG. 30. WALL OF THE YOLK-SAC IN THE REGION OF THE AREA OPACA OF A CHICK OF THE SECOND DAY. 

 Mes, Mesoderm. V,V, Blood-vessels, containing a few young blood-cells. Ent, Entoderm. c, Four distinctly 



shown entodermal cells. 



44i and 45. As shown in the section (Fig. 29), the yolk-sac, if we may so call it, 

 is completely enclosed by the somatopleure of the embryo, and in the amniote 

 embryo the condition is the same. The yolk-sac is surrounded by the somato- 

 pleure, which, however, in the amniota we call extra-embryonic. The extra- 

 embryonic somatopleure around the yolk-sac is called in birds the membrana serosa, 

 and in mammals the chorion. 



In amniota we can distinguish in the entoderm of the embryo, or yolk-sac, 

 three distinct regions. The first of these includes the whole of the entoderm of the 

 embryo and a certain territory around it. In this region, after the earliest stages 

 are passed, the entoderm is found to be a very thin layer and to contain very few 

 yolk-granules, and such few as it contains are small. This portion of the ento- 

 derm, therefore, seems translucent, an appearance which can easily be noted with 

 the naked eye, and which has led to the name area pellucida, which has long been 

 applied to this region. The region all around the area pellucida appears in the 

 fresh specimen darker, and this is called the area opaca, the second region. The 

 entoderm in this part consists of columnar cells (Fig. 30, c, and Fig. 31). In the 

 chick the cells are high cylinder cells of somewhat irregular shape, containing a 

 loose network of granular protoplasm. The lower ends of the cells are rounded 



