i8o 



CHAPTERS ON EVOLUTION. 



tntsoblast ; and an inner layer oikypoblast. It may be well to remark 

 at this stage of our inquiries the part played by each of these three 

 layers in the formation of the young animal. From the epiblast arise the 

 outer skin and the nervous system. The superficial layer of the body, 

 and the great internal nerve-centres governing the frame, its move- 

 ments and vital processes, thus arise from one and the same layer 

 a fact appearing to argue in favour of the origin of the nervous system 



FIG. 95. DEVELOPMENT OF CHICK. 



of Vertebrates from a layer which in anterior stages of existence (as in 

 the animalcules of to-day) originally formed the outer and sensitive 

 margin of the body. From the mesoblast or middle layer arise the 

 bones, muscles, blood-vessels, the under skin and other parts; whilst 

 the hypoblast or under layer gives origin to the 

 lining membrane of the digestive system, and to 

 such digestive organs as the liver, pancreas, &c. 



About the. sixth or eighth hour of incuba- 

 tion, these three layers of the blastoderm in the 

 chick are duly formed. Very rapidly succeed 

 the changes which result in the production of 

 the chick itself. A groove (Fig. 95, A, <:, and 

 93, E, e) soon appears on the surface of the 

 blastoderm, this furrow being known as the 

 " primitive groove," and constituting the keel 

 of the body, so to speak. The edges of the 

 groove finally grow together (Fig. 95, B, </), and 

 convert the groove into a canal (Fig. 94, c). A 

 portion of the epiblast is pinched off from the re- 

 maining portion, and being included within the 

 tube thus formed, duly gives origin to the brain and spinal cord. As 

 two projections of the blastoderm grow upwards to form the spinal 



