458 ZOOLOGY FOR MEDICAL STUDENTS chap. 



but the result of it is that the germinal disc is converted into a mass 

 of small cells — ^the blastoderm — ^which gradually increases in area so as 

 to spread over the surface of the yolk. The first obvious signs of 

 differentiation make their appearance towards the end of the first day 

 of incubation when the central-portion of the blastoderm (the pellucid 

 area) becomes marked off by its 'translucency from the surrounding 

 opaque area. The pellucid area, at first circular, becomes shaped like 

 the outline of a pear and along the axis of this towards the narrow end 

 runs a dim streak marked by a narrow groove along its centre — the 

 primitive streak and the primitive groove respectively. The study of 

 microscopic sections of this stage shows that the blastoderm consists 

 in its central portion of two distinct layers of cells. Of these the outer, 

 consisting of columnar cells closely fitted together, is the ectoderm, 

 while the inner gives rise in great part to endoderm. The primitive 

 ■streak is seen to be a region of active proliferation of the ectoderm. 

 Active cell-division is taking place and the cells so arising spread out- 

 wards on each side between the two primary layers as a sheet of mesoderm. 

 In certain parts the primitive streak shows complete cellular continuity 

 with the endoderm as well as with the ectoderm and the study of the 

 blastoderm of Reptiles gives a clue to the meaning of this for there are 

 indications that the primitive streak marks the position of the opening 

 of the gastrula which in the vertebrates has taken on a long slit-like 

 form and then become obliterated by fusion of its lips.^ 



Towards the end of the first day of incubation the ectoderm of the 

 pellucid area becomes raised up into a crease or fold in the form of a 

 long n, the open end of the n embracing the front end of the primitive 

 streak. This fold is commonly spoken of in the plural as the medullary 

 folds although the two folds are really quite continuous in front. The 

 study of transverse sections shows that the folds mark the edge of the 

 thickened area of ectoderm constituting the medullary plate — the first 

 rudiment of the central nervous system. As development proceeds the 

 medullary folds become more prominent and then arch inwards towards 

 one another and meet and fuse so that the medullary groove lying between 

 them becomes closed in to form the neural tube. 



In an egg opened at about the middle of the second day of incubation 

 (Fig. 192) the neural tube which has meanwhile increased considerably 

 in length is seen to be closed in except in its hinder portion. Further 

 its anterior portion has become distinctly dilated to form the rudiment 

 of the brain, the hinder more slender portion giving rise to the spinal 



* For discussion of this interesting theory see the present writer's Embryo- 

 logy, p. 493- 



