82 



THE EARLY DEVELOPMENT OF MAMMALS. 



/Ent 



EC 



Somatopleure and Splanchnopleure. 



The somatic mesoderm, together with the overlying ectoderm, constitutes the 

 somatopleure or primitive body-wall. The splanchnic mesoderm, together with the 

 underlying entoderm, constitutes the Splanchnopleure. The somatopleure and Splanch- 

 nopleure are, to a large degree, the elementary anatomical parts out of which the 

 adult structure is produced. Although they each comprise cells belonging to two 

 germ-layers, they nevertheless develop each almost as a unit, the cells of the two 

 germ-layers entering into intimate co-operation with one another in the differen- 

 tiation of organs. In both somatopleure and 

 Splanchnopleure it is convenient to distinguish two 

 main regions; namely, the embryonic, which enters 

 into the constitution of the embryo proper, and 

 the extra-embryonic, which enters into the forma- 

 tion of the so-called appendages of the embryo, 

 that is to say, of parts which exist during em- 

 bryonic life, but are lost at the time of birth, 

 and take no share in the permanent body. 



In the primitive type of vertebrate develop- 

 ment there are no embryonic appendages. This 

 condition is illustrated by figure 44, which is a 



transverse section of a young stage of an axolotl. 

 FIG. 44. TRANSVERSE SECTION OF AN * 



EARLY STAGE OF AN AXOLOTL. This may be readily compared with a blasto- 



Ec, Ectoderm, mes, Mesoderm. Md, dermic vesicle of a mammal, if we imagine the 



Medullary groove. Ch, Notochord. mags of }k Qr entoderm re duced to a single 

 Ent, Entoderm. Yk, Yolk. Ach, 

 Archenteron or primitive entodermal la J er of cells - We can then easil y distinguish the 



cavity. (After Bellond.) ectoderm and the underlying somatic mesoderm, 



which together completely enclose the section. 



The splanchnic mesoderm lies close against the yolk and is separated from the 

 somatic by the intervening coelom. 



The general homologies of this primitive type of vertebrate embryos with the 

 type which we find in the amniota may be readily grasped by the aid of the ac- 

 companying diagrams (Fig. 45), which are based somewhat on the processes as 

 actually found in the chick. The embryonic structures properly so called are dis- 

 tinguished by shading. The yolk-sac is large and more or less a separate structure 

 from the embryo. It is surrounded by a layer of mesoderm represented by a dotted 

 line. In the direction of the embryo the mesoderm has continued to form part of 

 the wall of the intestinal canal, In; hence we may say that the Splanchnopleure 

 forms the wall of the primitive intestinal canal and of the yolk-sac. The yolk-sac 

 represents a lower portion of the Splanchnopleure. It can be readily seen that we 

 may compare it with the condition noted in the newt, and have to deal funda- 

 mentally with a question of relative proportions. The somatopleure, Som, enters into 

 the formation of the embryo itself, but it also extends beyond. Its disposition be- 



