20 HISTOLOGY. 



As it spreads out around the vesicle it divides into two layers, one of which 

 is closely applied to the ectoderm (the somatic layer) and the other to the 

 entoderm (the splanchnic layer) . Between them is the body cavity or coelom, 

 which in the adult is subdivided into the peritoneal, pleural, and pericardial 

 cavities. The ectoderm and the somatic mesoderm constitute the somato- 

 plcure, or body wall; the entoderm and splanchnic mesoderm form the 

 intestinal wall, or splanchnopleure. The coelom has appeared in Fig. 19, 

 H, and in I it has attained a full development. On the ventral side of the 

 intestine it crosses the median line. Dorsally the medullary groove, 

 which has now become the medullary tube by the fusion of its upper 

 margins, separates the coelom into right and left portions. Fig. 19, I, 

 may be regarded as showing the fundamental relations to be observed in 

 the cross section of an adult, made through the abdominal cavity. 



Reviewing the preceding paragraphs it is seen that the fertilized 

 ovum through segmentation forms a morula, and later a blastodermic 

 vesicle composed of three germ layers, the ectoderm or outer, the mesoderm 

 or middle, and the entoderm or inner. For studying the transformation 

 of these layers into the organs and tissues of the adult, chick embryos 

 are more available than those of mammals. The structure of a chick 

 embryo of about thirty hours' incubation may therefore be briefly reviewed. 

 Fig. 20, A, represents a dorsal view of such an embryo, various portions 

 of which have been removed, and Fig. 20, B, is a median sagittal section of 

 a similar embryo. On the dorsal side the ectoderm forms a continuous 

 layer covering the embryo, and it becomes a part of the skin, the epi- 

 dermis and its appendages. In the figure (A) it has been cut away except 

 a portion folded in under the head and the part surrounding the rhomboidal 

 sinus, rh. s. Besides the epidermis the ectoderm forms the medullary 

 groove, the edges of which unite to form the tube beginning near the 

 head. The union of these edges proceeds in both directions. The 

 anterior neuropore is the last portion to close anteriorly (there are two 

 small anterior openings in B), and the rhomboidal sinus is the expanded 

 open part behind. Later these openings are closed over and the medul- 

 lary tube becomes detached from the epidermis. Its anterior part 

 enlarges to form the brain and the two optic vesicles (op. v.), each of which 

 becomes the retina of an eye. Its posterior part forms the spinal cord. 



The entoderm in dorsal view is the deepest layer, exposed by removing 

 the ectoderm and mesoderm. Under the head it forms a broad finger- 

 like pocket, the pharynx (ph.}. Its relations are seen in the median section. 

 Later its anterior end fuses with an inpocketing of the adjacent ectoderm 

 to form the oral plate. When this plate becomes thin and ruptures, the 

 pharynx opens to the exterior at the mouth. Posteriorly the entoderm 



