180 EMBRYOLOGY. 



from the yolk-containing part of the lower germ-layer, and become- 

 differentiated on the one hand into the migratory cells of the first 

 blood-vessels, and on the other into the blood-corpuscles. RUCKERT 

 further maintains that the material destined for the production of 

 blood is supplemented by means of cells freshly cleft off from (he 

 yolk. 



SWAEN remarks with the same positiveness, " Les premiers ilots 

 sanguins se developpent aux depens des elements de Thypoblaste. Ces 

 derniers constituent a la fin de ce developpement les parois de cavites 

 j vasculaires closes et les cellules sanguines qui les remplissent." 

 Likewise GENSCH makes the large cells in the yolk responsible for 

 the formation of the blood in the case of the Bony Fishes. HOFF- 

 MANN also finds in Reptiles that the blood and the endothelial 

 wall of the vessels, as well as the spindle-shaped cells which lie 

 between the vessels, are a product of the inner germ-layer, and 

 that they appear at definite places of the germ-disc at a time 

 when the middle germ-layer has not yet been formed in those 

 regions. 



Finally, it is stated concerning the germ of the Chick that at the- 

 end of the first day of incubation the cells in the yolk-wall have 

 become very numerous, through the multiplication of the nuclei 

 enclosed in the latter, and that afterwards the abundance of the 

 cells diminishes. For part of the cells which have been formed 

 by the active proliferation now detach themselves from the yolk- 

 wall, get into the space between the outer and inner germ-lay ers r 

 and there produce a third independent layer, which is continually 

 increasing in thickness, whereas the remaining part becomes modi- 

 fied into an epithelium of large cylindrical cells containing yolk- 

 granules. This middle layer is judged by several investigators to- 

 be an independent fundament of the germ, and has in this sense- 

 been described by His as parablast, by DISSE and others as vascular 

 layer, by RAUBER as desmohcemoblast, and by KOLLMANN as marginal 

 germ or acroblast. 



All of these accounts need still more precise confirmation, since- 

 they have often been called in question, even up to most recent 

 times. Thus KOLLIKEB has always defended the position that 

 not only the connective substances, but also the vessels and the- 

 blood, are products of the middle germ-layer, and are generated by it 

 in its peripheral regions. KASTSCHENKO, in his study of the Selachii, 

 could not convince himself that the merocytes have special import- 

 ance in the formation of blood and vessels, but was not, however,, 



