ORIGIN OF ORGANS. 69 



mesoblasts or cells which appear at an early stage between 

 the ectoderm and endoderm (e.g. in the earthworm's 

 development) ; or from numerous " mesenchyme " immi- 

 grant cells, which are separated from the walls of the blastula 

 or gastrula (e.g. in the development of Echinoderms) ; or 

 as coilom pouches outgrowths from the endodermic 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. The middle 

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

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

 and Coelentera 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 outgrowths 

 (e.g. the lungs, liver, allantois, etc., of higher Verte- 

 brates), also the axial rod or notochord. According 

 to some authorities, the blood and the vascular 

 system of Vertebrates are in the main endodermic 

 in origin. 



(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 perhaps 

 the vascular system. This layer aids in the forma- 

 tion of organs originated by the other two. With 

 it the reproductive organs are associated. 



