DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



139 



margin of the germ-disc in Birds and Reptiles, have an entirely dif- 

 ferent signification. This margin exhibits a very different relationship 

 from that of the primitive streak or blastopore ; it is a peculiarity of 

 meroblastic eggs, which is most intimately associated with the origin 

 of partial cleavage. It indicates the place at which the segmented 

 portion of the germ meets the unsegmented portion the place at 

 which there lie in the yolk free nuclei, by means of which a supple- 

 mentary cleavage is kept up until late stages in the process of 

 development, until, in fact, the time when the two primary germ- 

 layers have been formed by means of the invagination which 

 occurs at the blastopore. At the expense of the cell-material, which 

 is constantly being augmented by supplementary cleavage, the germ- 

 layers increase in extent at their place of transition into the yolk, 

 and thus gradually grow over the unsegmented part. Whereas at 

 the blastopore an invagination of cells already present takes place, there 

 ensues at the margin of the germ-disc a formation of new cells, and 

 thereby an increase of the marginal part and an overgrowth of the 

 yolk. I therefore propose for it the name circumcrescence-margin 

 of the yolk-sphere. There can be no such thing as a separate opening 

 or a yolk-blastopore, because the yolk is an organic part of the germ, 



and is in continuity 

 with the segmented 

 part of it by means 

 of the layer which 

 contains the yolk- 

 xiuclei. 



If we would insti- 

 tute a comparison be- 

 tween animals with 

 meroblastic eggs and 

 the Amphibia at a stage 

 when gastrulation is 

 not yet completed, then 

 the blastopore of the 

 Amphibia, which is 

 indicated by the letter 

 ^l in tile accompanying 

 section through the 



gastrula of a Triton (fig. 102), corresponds to the prostoma of Rep- 

 tiles, and to the crescentic and primitive grooves of Birds ; the still 

 exposed mass of yolk-cells corresponds to the yolk-material which is 



ak 



fh 



dz 



Fig. 102. Longitudinal section through a gastrula of Triton. 

 ak, Outer, ik, inner germ-layer ; fh, cleavage-cavity ; ud, coel- 



enteron ; u, blastopore ; dz, yolk-cells ; dl, dorsal, vl, 



ventral lip of the coelenteron. 



