ORIGIN OF ORGANS. 6i 



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 Ccelenterates 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 stomatodaeum and 

 hind-gut or proctodseum), and probably the segmental 

 or primary excretory duct. 

 {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 which pre- 

 cedes and is replaced by the backbone in all higher 

 Vertebrates and persists in a few of the lower. 

 (c) From the Mesoderm or Mesoblast arise all other struct- 

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

 skeleton, the lining of the body-cavity, and most 

 probably the vascular system. This layer aids in 

 the formation of organs originated by the other 

 two. With it the reproductive organs are assoc- 

 iated. 

 Physiological Embryology. — Of the physiological condi- 

 tions of development, we know relatively little. To investi- 

 gate 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 outfold- 

 ing which makes the coelome pouches, and what do the 



