■64 EMBRYOLOGY. 



Tho layer of yolk which immediately adjoins the periphery of the 

 ■cellnlar disc, and which is very finely granular and especially rich in 

 protoplasm, still merits particular consideration, for in it lie isolated 

 nuclei (fig. 35 nx), the much-discussed yolk-nuclei or parablast-nuclei 

 (the " merocytes " of Euckert). In the case of the Chick they are 

 less striking than in Teleosts and Selachians, in which they have 

 been accurately investigated by Balfour, Hoffmann,. Ruckert, 

 and Kastschenko. Formerly these were held to arise spontaneously 

 (free formation of nuclei) in the yolk, an assumption which in itself 

 is very improbable, since, according to our present knowledge, the 

 free formation of nuclei does not appear to occur anywhere in 



Jfig. 85. — Section through tho germ-disc of a Pristiurus embryo during segmca^ation, after 



Balfour. 

 It, Nucleus; tix, modified nucleus prior to dirision ; 'tix\ modified nucleus in the yolk; /, 



furrows which appear in the yolk adjacent to the germ-disc, 



•either animal or vegetable kingdom. Consequently the yolk-nuclei 

 are now rightly held to he derived from the cleavage-nuclei. They 

 are probably produced even at an early period, when the first-formed 

 segments, which remain, as we have seen, for a long time in connection 

 with the yolk, begin to be constricted ofi" from the latter. This 

 probably takes place in the following manner : there arise in the 

 segments nuclear spindles, the halves of which go into the completely 

 isolated embryonic cells at the time of their separation from the 

 yolk, while the remaining halves go into the underlying yolk-layer, 

 and are there converted into vesicular yolk-nuclei. 



Their number subsequently increases by means of indirect division, 

 as is established by the fact that in sections nuclear spindles have 

 been observed in the yolk-layer (fig. 35 nx'). 



While, on the one hand, there is an increase in the number of the 

 yolk-nuclei, so, on the other hand, there is also a diminution in their 



