EMBRYOLOGY 



II 



henceforth known as a coelom^ or body cavity (fig. 6). Each coelo- 

 mic space has two walls, one toward the ectoderm, the somatic layer, 

 the one toward the entoderm being the splanchnic layer (figs. 5 

 and 6, so, sp). 



The mesoderm arising in this way and bounding the coelom is 

 (ailed mesotheliiun to distinguish it from another kind — the mesen- 

 chyme — which also comes to lie in the segmentation cavity. This 

 mesenchyme arises as separate cells, coming largely from the meso- 

 thelium, and to a less extent from the entoderm (see p. 20). Whether 

 any arises from the ectoderm is disputed. 



IiG. 5. Fig. 6. 



Fig. 5. — Stereogram of the anterior end of a developing amphibian, showing the out- 

 lining of the mesothelium, nervous system and notochord. a, anterior end; ar, archen- 

 teron; c, coelom; ch, notochordal cells; ec, ectoderm; mp, mesodermal pouch; ng, primi- 

 tive groove; np, neural plate; nr, neural folds; sc, segmentation cavity; so, somatic wall 

 of coelom; sp, splanchnic wall of coelom. 



Fig. 6. — Stereogram of the anterior end of a vertebrate, showing the relation of the 

 coelomic pouches; c, coelom; d, digestive tract; e, ectoderm; nc, nervous system; n, 

 notochord; sc, segmentation cavity; so, somatic and sp, splanchnic walls. 



The Germ Layers. — Ectoderm, entoderm and the two types of 



mesoderm are called the germ layers, because in the animals first 



tudied they were arranged like layers one on the other. Each plays 



its part in the formation of the adult and gives rise to its peculiar 



structures. 



The ectoderm forms the outer layer of the skin, hair, claws, fea- 

 thers, the outer layer of scales, enamel of teeth, and the essential or 

 ( haracteristic part of all sensory and nervous structures. 



The entoderm gives rise to the lining of the digestive tract, and 

 the various outgrowths — gills, lungs, liver, pancreas, etc. — connected 

 •vith it. The notochord is also entodermal and possibly the lining 

 uf the blood-vessels is derived from this layer. 



* A coelom formed in this way is an enterocoele. Usually the coelomic walls arise 

 as a solid mass of cells from the corresponding region, which later splits internally, 

 forming a schizocoele. The two are readily compared. 



