BY DR. KLEIN. 167 



per cent, bichromate of potash, and one part glycerin. The 

 cover-glass is fixed by means of sealing-wax. 



Sections through the unincubated germ-disk show that it 

 consists of two layers, besides the formative elements which 

 are to be found on the floor of the cleavage-cavity, and at the 

 yolk-rim. (See fig. 176.) Sections made during the first half 

 of the first day teach that these formative elements find their 

 way from the yolk-rim in between the two la} r ers of the germ, 

 so as to form, first (seventeenth hour), the central part of the 

 middle layer of the area pellucida, and afterwards (at the 

 twenty-third or twenty-fourth hour), the remaining portion of 

 that layer. Thus, at the end of the first day, the germ-disk, 

 which before consisted of two layers, consists in the area pellu- 

 cida of three upper, lower, and middle the last originating 

 from the formative elements which had previously rested on 

 the floor of the cleavage-cavity. 1 



As the central nervous system is developed from the central 

 portion of the upper layer, the remainder of this layer giving 

 rise to the epithelium of the skin and of the cutaneous glands, 

 it follows that the upper layer in the chick represents the upper 

 and nervous layers in fish and Batrachia ; it is therefore simply 

 called corneal layer: the middle layer in the chick corresponds 

 to the third in the trout and in Batrachia, and is therefore 

 termed motor-germinative ; the lowest layer in the chick cor- 

 responds to the fourth in the germ of trout and Batrachia, and 

 is termed the epithelial glandular layer. 



When the central part of the middle germinal layer is formed 

 (seventeenth hour), the upper one is seen to be thickened at its 

 middle portion ; it consists of cylindrical cells. At the same 

 time, this middle portion of the upper layer is more or less 

 fused with the just deposited central part of the middle ^er. 

 This condition shows on a surface view the primitive streak 

 (Axenslrang). Along with the formation of the primitive 

 streak, the dorsal groove is also developed, a differentiation of 

 the middle layer of the germ takes place into the notochord 

 and protovertebroe, and the dorsal lamina begin to project. 

 In the first hours of the second day of incubation, the dorsal 

 laminae are seen to be already approaching one another, so that 

 in the region of the neck they almost touch ; at the tail end 

 they are still a considerable distance apart, so that the dorsal 

 furrow is very shallow. A short time afterwards, the dorsal 

 laminae in the cervical region are observed to be completely 

 closed, and the dorsal furrow is changed into a canal the cen- 

 tral canal of the central nervous system. (Figs. 177, 178.) 



In sections made later in the second da}^ of incubation, the 



1 The corneal, motor-gcrminative, and epithelial glandular layers cor- 

 respond to the epiblast, mesoblast, and hypoblast of Huxley. ED. 



