GENERALISATIONS. 67 



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 endo- 

 derm, 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 proctodeum). 

 (0) 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. 



Physiological embiyology. — Of the physiological conditions of develop- 

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

 tasks of the future. Why does an egg cell form polar bodies, how is 

 the sperm attracted to the ovum, why does the fertilised egg cell divide, 

 how does the yolk affect segmentation, what are the conditions of the 

 infolding which forms the endoderm, and of the outfolding which makes 

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



Generalisations. — (1) The ovum theory or cell theory. — 

 All many-celled animals, produced by sexual reproduction, 

 begin at the beginning again. " The Metazoa begin where 

 the Protozoa leave off" — as single cells. Fertilisation does 

 not make the egg cell double ; there is only a more com- 

 plex and more vital nucleus than before. All development 

 takes place by the division of this fertilised egg cell and its 

 descendent cells. 



