DEVELOPMENT OP CONNECTIVE SUBSTANCE AND BLOOD. 



179 



Toward the surface of the yolk small clusters of nuclei (fig. 113 k) 

 arise out of the large deeper-lying yolk-nuclei. From these there 

 are finally produced genuine cells of the germ (z), by the small nuclei 

 surrounded by a layer of protoplasm detaching themselves from 

 the yolk, as it were by an act of supplementary cleavage. " Since the 

 merocytes thus on 

 the one hand un- 

 interruptedly take 

 up nutritive ma- 

 terial out of the 

 yolk, and on the 

 other continually 

 surrender it in the 

 form of cells to the 

 germ-layers of the 

 nascent embryo, 

 they present an 

 important link 

 between the latter 

 and the yolk" 

 (RUCKERT.) 



The views of 

 investigators on 

 the significance 



of the yolk- wall and of the merocytes enclosed in it are very divergent. 

 Indeed there is unanimity only in this, that the yolk-wall contributes 

 to the increase of the lower germ-layer by single cells becoming in- 

 dependent and attaching themselves at the margin to the elements 

 which already have an epithelial arrangement. On the other 

 hand it appears less certain how far the yolk-wall is concerned in 

 the formation- of the blood. According to the observations of His, 

 DISSE, RAUBER, KOLLMANN, RUCKERT, SWAEN, GENSCH, HOFFMANN, 

 and others, it does share in this process during a limited period 

 of development in the case of Selachians, Teleosts, Reptiles, and 

 Birds. 



In the Selachians the anterior margin of the germ-disc is the first 

 to be metamorphosed into a vascular zone. RUCKERT could find 

 here numerous and unequivocal indications that the previously 

 described peculiar cell-elements of the yolk (merocytes) provided 

 with large nuclei contribute to the formation of blood-islands, in 

 that they break up into clusters of small cells, detach themselves 



Fig. 113. Yolk-nuolei (merocyte) from Pristiurus, lying underneath 

 the germ-cavity B, after RUCKERT. 



z, Embryonic cells ; k, superficial clear nuclei ; k l t deeper nuclei ; 

 k* t marginal nuclei rich in chromatin, largely freed from th 

 surrounding yolk, in order to show the processes of the proto- 

 plasmic mantle ; d, yolk-plates. 



