190 EMBRYOLOGY. 



All fundaments of the animal body are derived from embryonic cells, which 

 have been produced from the egg-cell by the process of cleavage. The dis- 

 tinction between middle germ-layer and mesenchyme-germ is to be sought 

 in another direction than in that indicated by His. The middle germ-layers 

 are sheets of embryonic cells, having an epithelial arrangement, which arise by 

 a process of folding from the inner germ-layer, just as the latter does by a fold- 

 ing of the blastula (compare the historical part of Chapter VII.). The mesen- 

 chymatic germ, on the contrary, embraces cells, which have been individually 

 detached from epithelial union in the inner germ-layer, and furnish the founda- 

 tion for connective substance and blood by spreading themselves out in the 

 system of spaces between the epithelial germ-layers. 



After the appearance of the Ccelom-Theory, His entered again into an 

 explanation of his parablast-theory, and modified it in his paper, " Die Lehre 

 vom Bindesubstanzkeim," in so far as he no longer laid weight on the 

 question whether the fundament of the connective substance was derived from 

 the segmented or the unsegmented germ. 



The theory of the double origin of tlie middle germ-layers, established by 

 His and by us in different ways, met with opposition on the part of KOLLIKER 

 who held to the older interpretation ; but by many others it was accepted ; 

 attempts were made further to confirm and also to modify it by KUPFFER, 

 DISSE, WALDEYER, KOLLMANN, HEAPE, and others, who defended the existence 

 of a special connective-tissue germ. 



KUPFFER and his followers furnished important observations concerning" 

 the presence of yolk-nuclei in a definite zone of the embryonic fundament, and 

 their relation to the formation of blood in Fishes and Reptiles. 



HOFFMANN and KtiCKERT showed that the yolk -nuclei do not arise by free 

 [spontaneous] formation of nuclei, but are descendants of the cleavage-nucleus. 



DISSE investigated the germ-wall of the Hen's egg. 



KOLLMANN named the cells which migrate out between the germ-layers 

 poreuts (Poreuten), and the whole fundament the acroblast. 



Finally, WALDEYER endeavored to derive the connective-tissue germ from 

 a special part of the cleavage-material, which he divided into an archiblast 

 and a parablast. 



According to WALDEYER'S theory, the cleavage of the eggs of all those 

 animals in which there is any blood and connective substance does not take 

 place uniformly up to the end, but one must distinguish a primary and a 

 secondary cleavage. " The former divides the egg, so far as it is in any way 

 capable of cleavage, into a number of cells, which are ready for the production 

 of tissues. These then form the primary germ-layers. A remnant of im- 

 mature cleavage -cells (in the case of holoblastic eggs), or of egg-protoplasm r 

 which is not yet converted into the cell-form (in meroblastic eggs), is left 

 remaining. Neither the immature cells, nor the protoplasm still unconverted 

 into cells, enter for the present into the integrating condition of the germ- 

 layers. On the contrary, it is only afterwards that there is effected on this 

 material a further formation of cells, the secondary cleavage. The immature 

 cells of the holoblastic eggs, over-loaded with nutritive yolk, divide them- 

 selves, or, if one prefers, ' cleave ' themselves further, or the parts which are 

 most richly provided with protoplasm constrict themselves off from the 

 eggs, whereas the remnant of the nutritive material is consumed, the 

 unformed remnants of the protoplasm (germ-processes) of meroblastic eggs 

 become divided up into cells. The cell-material thus secondarily acquired 



