THE NEltVOUS SYSTEM 117 



ever, mainly at the expense of the yolk-cells, with which it 

 is in contact. By growth at its margin it gradually creeps 

 round until it extends all round the embryo, the yolk-cells 

 forming part of its ventral wall. 



Fate of the germinal layers. From one or other of the 

 three germinal layers epiblast, mesoblast, and hypoblast 

 all parts of the embryo are formed. 



The epiblast, or outer layer, gives rise to the epidermis 

 covering the body generally, and to the various glandular and 

 other structures derived from the epidermis ; to the nervous 

 system, both central and peripheral ; to the olfactory and 

 auditory epithelium, to the retina and lens of the eye, and to 

 the other sensory organs ; to the epithelial lining of the mouth 

 and anus (stomodaeum and proctodseurn) ; and to the pineal 

 and pituitary bodies. 



The hypoblast, or inner layer, gives rise to the epithelium 

 lining the alimentary canal and its various diverticula, includ- 

 ing the glands of the oesophagus, stomach, and intestine, the 

 lungs, the bladder, the bile ducts, gall-bladder, pancreatic 

 ducts, and the hepatic cells of the liver and the secreting 

 cells of the pancreas; the notochord is also formed from 

 hypoblast. 



From the mesoblast, or middle layer, are derived all struc- 

 tures between the epiblast and hypoblast ; i.e., the connective 

 tissue, muscles, skeleton (except the notochord), bloodvessels 

 and lymphatics ; and also the peritoneum, and the urinary 

 and reproductive organs. 



F. Development of the Nervous Sj stem. 



It is convenient from the point we have now reached to 

 deal with the several systems one by one. The nervous 

 system is a suitable one to commence with, as it appears at a 

 very early stage of development, and plays an important part, 

 especially in the younger stages, in determining the shape 

 and proportions of the embryo. 



The epiblast consists almost from the first of two layers, the 

 distinction between which is already established at the close of 

 segmentation. (Fig. 23.) Of these the upper or epidermic 

 layer is a single stratum of closely fitting cubical cells ; while 



