ORIGIN OF ORGANS 83 



between the ectoderm and endoderm {e.s;. in the earth- 

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

 immis^rant cells, which are separated from the walls of the 

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

 derms) ; or as ccelom pouches — outgrowths from the en- 

 dodermic lining of the gastrula cavity {e.g. in Sagitia, 

 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, theother(visceral or splanch- 

 nic) 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, w^hich 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 stomodasum 

 and a trace of hind-gut or proctodasum). 

 {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 

 the reproductive organs are associated. Con- 

 nective tissues, vascular system, and unstriped 

 muscles are formed by mesenchyme cells which 

 are budded off from the true mesoderm. 



