"PMEXT OF CONNECTIVE K AM) BLOOD. 



170 



other 





Toward the surface of the yolk small 1 _. 1 13 k) 



out of the large deeper lyii.ir yolk nuclei. Fn>m these there 



are finally produced genuine cell . ). by t ho small nuclei 



surrounded by a layer of protoplasm detaching themselves from 



.oik, as it \\eir by an act of .supplementary cleavage. " Since t/ie 



y-.v thus on 



" li<t,l UU- 

 interri'j't'-'l/y take 

 iijt nitiri 



t'-riil cut of the 

 ijulk. innl on the 

 anally 

 in the 



of cells to the 

 n i >f (//>> 



nascent embryo, Vr;". 



, . 



they present an 



important link ^ 



between the latter 



and the yolk." 



(RUCKERT.) 



The views of 

 in ve-ii gators 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 



ndent and attaching themselves at the margin to the elements 

 which already have an epithelial arrangement. On the ol 

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

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



DlSSE, 1\AI I5KH, KOLLMANN, RUCKERT, SWAEN, GfiNSCH, Hoi IMA NN. 



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



of development in the case of Selachian.-. Teleo>ts. Reptiles, and 



Birds, 



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

 to be metamorphosed into a vascular zone. li ould find 



here numerous and unequivocal indications that the previously 

 described peculiar cell-elements of the yolk (n provided 



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

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



Fig. 113. Yolk-nuclei (merocytes) from Pristiuriu, lying underneath 

 the germ-cavity B, after RCCKF.RT. 



:, Kin>>iyonic cells; k, superficial clear nuclei; i 1 , deeper nuclei; 

 *, m:uin.il niul.-i rich in chnmuitin, largely freed frum the 

 surrouiidui- \.-lk. in order to show the processes of the proto- 

 plasmic mantle ; d, yolk-plates. 



