“7O REPRODUCTION AND LIFE HISTORY. 
most essential parts of the sense organs, infoldings 
at either end of the gut (fore-gut or stomodeum 
and hind-gut or proctodzeum). 
(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, ¢.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. 
Physiological embryology.—Of the physiological conditions of develop- 
ment we know relatively little. To investigate them is one of the 
tasks of the future. Why does the fertilised egg-cell divide, how does 
the yolk affect segmentation, what are the conditions of the infolding 
Fic. 33.-—Embryos—(1) of bird ; (2) of man.—After His. 
The latter about twenty-seven days old. 
y.s., Yolk-sac ; A2., placenta. 
which forms the endoderm, and of the outfolding which makes the 
coelom pouches ; in short, what are the immediate conditions of each 
step in the familiar process by which, out of apparent simplicity, cbvious 
complexity arises? 
Generalisations.—(1) Zhe ovum theory or cell theory.— 
All many-celled animals, produced by sexual reproduction, 
