DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 



179 



-O-dL 



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

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

 are finally produced genuine cells of the germ (»), 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 linh 

 between the latter 

 and the yolk." 

 (fiucKEBT.) 



The views of 

 investigators on 

 the significance 



of the yolk-wall and of the merocytes enclosed ia 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, Raubee, Kollmann, RiJCKERT, 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 (merocytes) from Pristiurus, lying underneath 

 the germ-cavity B, after B'dokert. 



z, Embryonic cells ; k, superficial clear nuclei ; k^, deeper nuclei ; 

 k*, marginal nuclei rich in chromatin, largely freed from ths' 

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

 plasmic mantle ; d, yolk-plates. 



