1014 



EMBRYOLOGY. 



cutaneous layer lining the ectoderm constitutes the somatopleure ; and the fibro- 

 intestinal layer lining the endoderm constitutes the splamhnopleure. 



But it is essential to remark that phenomena of the same order occur pro- 

 gressively in the extra-embryonal mesoderm, which, throughout its extent, 

 undergoes delamination, and becomes two layers equally aUied with the 

 ectoderm and endoderm. It is sufficient to add that the somatopleure and 

 splanchnopleure are continued beyond the embryo by two identical formations, 

 which it is convenient to designate as the extra-embryonal somatopleure and 

 splanchnopleure. 



We have given all the facts and adopted the language which, up to the 

 present, can be embraced in a preliminary view of the general direction of the 

 development and the manner in which the vertebral type is realized, as well as 



TRANSVERSE SECTION OF THE DORSAL REGION OF AN EMBRYO CHICK OF FORTY-FIVE HOURS. 



A. Ectoderm ; B, mesoderm ; M.c, neuraxis ; P.v., lateral layer, or mesoblastic limit ; p.p., pleuro- 

 peritoneal cavity ; c.h., notochord ; S.o., somatopleura ; S.p., splanchnopleura ; W.d., Wolffian 

 canal; a.o., aorta; v, blood-vessels; ic, mesodermic rid,;e, or germinal wall; op, limit of the 

 opaque area and area pellucida. The figure repi'eseats the right half of the section. 



the correlations between the embryonal and extra-embryonal blastoderms. The 

 study of these correlations allows us to trace the origin of the envelopes which 

 protect the foetus, and are known as the annexes ofthefaitus. 



In Fig. 553 is given a series of sections of an embryo more or less advanced. 

 In sections F and I the embryonal somatopleui-e and splanchnopleure are already 

 incurvated downwards and inwards. In passing from F to I, we see how the 

 splanchnopleure of each side advance towards each other, and tend to form a 

 tube opened inferiorly at each section — this is the intestine. Its walls, not 

 joined at this point, are continuous with the extra-embryonal splanchnopleure, 

 which is really the umbilical reside ,• in Birds, the latter contains the mass of 

 the yolk. Above the intestine are seen the superposed sections of the notochord 

 and neural canal ; and it is the order of this superposition that is characteristic 



