68 REPRODUCTION AND LtFE HISl^ORY OF ANIMALS. 



tween 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 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 : — 



(rt) From the Edoderui 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 stomatodjeum 

 and hind gut or proctodeum), and perhaps the 

 segmental or primary excretory duct. 

 (/') From the Endoderm or Hypoblast arise the mid gut 

 (mesenteron) and the foundations of its outgrowths 

 (e.g., the lungs, liver, allantois, iS:c., of higher Verte- 

 brates), also the axial rod or notochord. According 

 to some authorities, the blood and the vascular 

 system of Vertebrates is 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. ^^'ith it 

 the reproductive organs are associated. 



P/iysiiIi\i;i,-al Eiii/iiyo!ox)'.^0(ihe physiological conditions of develop- 

 ment, we know relatively little. To investignte them, is one of the 

 tasks of the fiitiu'e. Why does an egg cell form polar bodies, how is 

 the sjierm attracted to the ovum, why does the fertilised egi; cell divide, 

 how does the yolk affect segmentation, what are the coni!lTtions of the' 

 infolding which forms the endoderm, and of the oiitfoldino- which makes 

 the coelome pouches, and what do the numerous larval stages mean ? 



