24 INTRODUCTION. 



nails, claws, and other epidermal modifications ; and the epi- 

 thelial lining of the mammary, sweat, and other glands formed 

 from the skin. 



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

 lining the alimentary canal and its various diverticula ; inclu- 

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

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

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

 the pancreas. The notochord also is formed from hypoblast. 



TP 



RT 



Cli 



FIG. 10. Transverse section through the head of a Chick Embryo at the 

 end of the first day of incubation, showing the relations of the three 

 germinal layers, x 100. 



B, cavity of the brain : the origin of the walls of the brain from the epiblast is well 

 seen. CH, notochord, arising from the hypoblast. E, epiblast. H, hypoblast. 

 IN" A, root of one of the cranial nerves. TP, cavity of the alimentary canal, in the 

 pharyngeal region. RT, blood-vessel. The whole of the part of the figure covered 

 by the lighter shading is mesoblast. 



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

 tures lying between the epiblast and hypoblast ; i.e. the con- 

 nective tissue, muscles, skeleton (except the notochord), blood- 

 vessels, and lymphatics ; and also the peritoneum, and the 

 urinary and reproductive organs. 



The General History of Development : the Recapitulation Theory. 



It is a familiar fact that animals in the earlier stages of their 

 existence differ greatly in form, in structure, and in habits from 

 the adult condition. 



In some cases, as, for example, in Amphioxus, the whole 

 history of development is a steady upward progress towards the 

 adult condition, the several organs and parts gradually approxi- 



