THE NERVOUS SYSTEM 115 



like the epiblast, from which it is derived, consists of small 

 cells containing pigment : while the yoLlc hypoblast cells are 

 larger and have no pigment. The invaginate hypoblast forms 

 the notochord and the roof of the alimentary canal, while the 

 yolk hypoblast gives rise to its sides and floor. The line between 

 the two is at first a fairly sharp one, especially at the hinder 

 end, near the blastopore : later on it disappears. 



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 nervovis 

 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 movith 

 and anus (stomodseum and proctodajum) ; 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, including 

 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 System. 



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 



