86 



THE EARLY DEVELOPMENT OF MAMMALS. 



granules, and containing conspicuously large nuclei. These large nuclei differ 

 by their size and minute structure very much from the other nuclei in the em- 

 bryo. The corresponding nuclei in higher animals are sometimes called para- 

 blast 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 illus- 

 trated by Fig. 34, 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 Figs. 30, 34, and 31. As shown in 

 the section, Fig. 34, 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 



Ent 



FIG. 35. WALL OF THE YOLK-SAC IN THE REGION OF THE AREA OPACA OF A CHICK OF THE SECOND DAY. 

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



shown entodermal cells. 



the same. The yolk-sac is surrounded by the somatopleure, 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 entoderm, 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 



