124 



DEVELOPMENT OF THE FROG. 



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 ; 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), blood-vessels 

 ^nd 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 



N.G 



Fig. 27. Transverse section through a frog embryo during the for- 

 mation of the neural canal. . 



C, ccelom : EE, epidermic layer of epiblast : EN, nervous layer ot 

 epiblast: H, hypoblast: M, mesoblast: ME, somatopleuric layer of meso- 

 blast : MH, splanchnopleuric layer of mesoblast: MN, mesenteron : 

 IM'r notochord : NF, neural fold : NG, neural groove : Y, yolk cells. 



