178 



EMBRYOLOGY. 



the margin of the disc. In the area pellucida and in the immediately 

 surrounding parts it appears as a single layer of greatly flattened 

 cells, and is separated from the yolk-floor by a cavity filled with an 

 albuminous fluid ; in the opaque area it reposes directly upon the 

 yolk ; its cells here become higher, cubical, or polygonal, and finally 

 it terminates with a greatly thickened marginal zone, the previously 

 mentioned yolk- wall (dw). This is the important region of the germ 

 with which we now have especially to deal. 



The yolk-wall consists in the Chick partly of embryonic cells, 

 which are separable from one another, partly of yolk-material 



in which are enclosed 

 numerous large and 

 small nuclei enveloped 

 in protoplasm (the me- 

 rocytes), as at the final 

 stages of the process of 

 cleavage. 



Such free nuclei have 

 also been demonstrated 

 with perfect certainty 

 in the marginal terri- 

 tory of the yolk during 



the course of the formation of the germ-layers in Selachians, 

 Teleosts, and Reptiles (KUPFFEE, HOFFMANN, RUCKERT, STRAHL, 

 SWAEN). 



The most accurate description of the yolk-nuclei has been given by 

 RUCKERT for the eggs of Selachians (fig. 113). They are present in 

 this case at the marginal portion of the germ-disc, embedded in the 

 yolk in not inconsiderable numbers, and are remarkable for their 

 size, sometimes reaching a diameter ten-fold as great as that of an 

 ordinary nucleus (k\ k*). From the protoplasm enveloping the 

 nucleus k* there proceeds a richly branched network of processes. 

 In the interstices of the net are lodged yolk-elements (d) in great 

 numbers, from the size of the ordinary yolk-plates down to the finest 

 granules. The former are often in process of disintegration. One 

 may conclude from this, as well as from other phenomena, that a 

 vigorous consumption of deutoplasm is taking place at the margin of 

 the germ. This deutoplasm is taken up as nutritive material by the 

 protoplasmic net surrounding the nucleus, and employed by means of 

 intra-cellular digestion for its growth. Consequently one also sees the 

 yolk-nuclei in active increase. 



Tig. 118. Section through the margin of the germinal 

 disc of a Hen's egg incubated for six hours, after Du VAL. 



<ilc, Outer germ-layer ; dz, yolk-cells ; dk, yolk-nuclei ; 

 dw, yolk-wall. 



