160 



INTERNAL FACTORS 



IV. i 



produced by the overgrowth of the blastoporic lip was parallel 

 to the plane of separation : the yolk-cells were pushed into the 

 segmentation cavity. The Hemiembryo had half a medullary 



FIG. 76.- A and B. Meridional sections through Hemiblastulae of the 

 Frog : Ten, remains of nucleus ; k', large reticular nuclei ; k, nuclei in 

 the yolk ; F, blastocoel ; r, vacuoles. C. Hemigastrula lateralis. Oblique 

 longitudinal section. D. Hemiembryo sinister, transverse section ; ss, 

 median plane ; the right half of the egg'is already completely cellulated, 

 post-generation of the germ-layers has begun ; ch, notochord, of whole 

 size ; mw, medullary fold ; u, gut ; j, two yolk-cells that have remained 

 embryonic. (From Korschelt and Heider, after Roux.) 



tube, mesoderm on one side, half a gut cavity open towards the 

 dead blastomere but a whole notochord (attributed by Roux to 

 premature regeneration). 



