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KEGENERA TION 



Hertwig repeated Roux's experiment and obtained results entirely 

 different from those of Roux. He injured one of the first two blasto- 

 meres of the frog's egg with a hot needle, or by means of a galvanic 

 current. Hertwig states that after the operation the egg turns so 

 that the uninjured part lies uppermost. This is owing, he thinks, to 

 the appearance of a blastula or of a gastrula cavity in the developing 



FIG. 62. After O. Hertwig. A. Section through a frog's egg (btastula stage) in which one hlasto- 

 mere had been killed. B, Same. Gastrula stage. C. Later gastrula stage. D, E. Surface 

 view of embryos from one of first two blastomeres. F. Same as last (). Dorsal view. 

 G. Ventral view of last. H. Dorsal view of another embryo, lying in a very eccentric position. 

 /. Later stage of embryo from one blastomere. Other injured blastomere nearly covered 

 over. J. Section through gastrula stage of embryo from one of first two blastomeres. 

 K. Cross-section of the embryo shown in F and G. 



part. The segmentation cavity is found in many cases surrounded by 

 the cells of the segmenting half (Fig. 62, A), but at other times at the 

 border between the new and the old parts. In still other cases the 

 cavity may lie eccentrically, and in some cases the floor of the cavity 

 may be bounded by the yolk substance of the injured half. An em- 

 bryo appears on the upper, uninjured part, though it is not, according to 

 Hertwig, a half-embryo, but a whole embryo, or at least one approach- 



