ORIGIN OF ORGANS. 69 



and into which cells wander from these two layers. In the 

 other Metazoa, the middle layer may arise from a few 

 primary mesoblasts or cells which appear at an early stage 

 between the ectoderm and endoderm (e.g. in the earth- 

 worm's development) ; or from numerous " mesenchyme " 

 immigrant cells, which are separated from the walls of the 

 blastula or gastrula (e.g. in the development of Echino- 

 derms) ; or as calom pouches outgrowths from the en- 

 dodermic lining of the gastrula cavity (e.g. in Sagitta, 

 Balanoglossus, Amphioxus)-, or by combinations of these 

 and other modes of origin. The mesoderm lies or comes 

 to lie between ectoderm and endoderm, and it lines the 

 body cavity, one layer of mesoderm (parietal or somatic) 

 clinging to the ectodermic external wall, the other (visceral 

 or splanchnic) cleaving to the endodermic gut and its 

 outgrowths. 



Origin of organs. From the outer ectoderm and inner 

 endoderm, those organs arise which are consonant with the 

 position of these two layers, thus nervous system from the 

 ectoderm, digestive gut from the endoderm. T.he middle 

 layer, which begins to be developed in " Worms," assumes 

 some of the functions, e.g. contractility, which in Sponges 

 and Ccelentera are possessed by ectoderm and endoderm, 

 the only two layers distinctly represented in these classes. 



In a backboned animal the embryological origin of the 

 organs is as follows : 



(a) From the ectoderm or epiblast arise the epidermis 



and epidermic outgrowths, the nervous system, the 

 most essential parts of the sense organs, infoldings 

 at either end of the gut (fore-gut or stomodaeum 

 and hind-gut or proctodaeum). 



(b) From the endoderm or hypoblast arise the mid-gut 



(mesenteron) and the foundations of its out- 

 growths (e.g. the lungs, liver, allantois, etc., of 

 higher Vertebrates), also the axial rod or noto- 

 chord. 



(c) From the mesoderm or mesoblast arise all other struc- 



tures, e.g. dermis, muscles, connective tissue, bony 

 skeleton, the lining of the body cavity, and the 

 vascular system. This layer aids in the formation 

 of organs originated by the other two. With it 



