SOMATOPLEURE AND SPLANCHNOPLEURE. 



77 



as a unit, the cells of the two germ-layers entering into intimate co-operation 

 with one another in the differentiation 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 formation of the so-called appendages 

 of the embryo, that is to say, of parts which exist during embryonic life, but are 

 lost at the time of birth, and take no share in the permanent body. 



In the primitive tyj>e of vertebrate development there are no embryonic 

 appendages. This condition is illustrated by Fig. 30, which is a transverse 

 section of a young stage of an axolotl. This may be readily compared with a 

 blastodermic vesicle of a mammal, if we im- 

 agine the mass of yolk or entoderm reduced 

 to a single layer of cells. We can then easily 

 distinguish the ectoderm and the underlying 

 somatic mesoderm, which together completely 

 enclose the section. The splanchnic meso- 

 derm lies close against the yolk and is separated 

 from the somatic by the intervening ccelom. 



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 accompanying diagrams (Fig. 

 31), which are based somewhat on the processes 

 as actually found in the chick. The embryonic 

 structures properly so called are distinguished 

 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 repre- 

 sented 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 splanch- 

 nopleure 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 fundamentally with a question of relative proportions. 

 The somatopleure, Som, enters into the formation of the embryo itself, but it 

 also extends beyond. Its disposition becomes complicated in the amniota by 

 the formation of the amnion itself. We shall consider here only what is looked 

 upon as the primitive method of the production of the amnion, and note only 



Fig. 30. — Transverse Section of an 

 Early Stage of an Axolotl. 



£c, Ectoderm. mes, Mesoderm. Aid, 

 Medullary groove. C/i, Notochord. 

 £n^. Entoderm. Yk, Yolk. Ac^, 

 Archenteron or primitive entodermal 

 cavity. — {After Bellonci.') 



