170 INTERNAL FACTORS IV. i 



the living portion, not at the edge ; the blastopore is usually 

 symmetrical to the plane of separation of the two blastomeres. 

 Beneath the lip of the blastopere an archenteron is formed, and 

 notochord and mesoderm are differentiated. The closure of the 

 blastopore is, however, prevented by the resistance offered by the 

 dead yolk-mass, which lies ventrally and posteriorly, just as it is 

 retarded by the yolk in a large-yolked Fish egg ; in fact, were this 

 dead yolk removed, the living portion, Hertwig maintains, would 

 develop normally. A nearly normal embryo may, in fact, be 

 formed, but more usually there are considerable abnormalities. 

 The mass of dead yolk, though partially enclosed by the grow- 

 ing edge of the ectoderm, is sufficiently great to impede the 

 ultimate closure of the blastopore, and sometimes of the 

 medullary folds as well; the latter frequently diverge round 

 the yolk -plug of dead and living tissue, the chorda being split 

 as well; they may be symmetrical, or one side may be much 

 less developed than the other, owing, it is asserted, to an 

 asymmetry in the resistance offered by the dead cell, and in 

 the more extreme cases this inequality may be so pronounced 

 as to give rise to a condition which does not differ in any way 

 from Roux's Hemiembryo lateralis; there is one medullary 

 fold, a notochord, mesoderm on one side, and a gut cavity in 

 the yolk-mass; to the bare side of the yolk-cells is attached 

 the dead blastomere, only partially covered by extensions of the 

 ectoderm of the living half. Such cases are, however, very rare ; 

 in the majority more, sometimes much more, than a half is 

 produced from the living blastomere, for Hertwig denies that 

 the missing parts are post-generated, as Roux maintains, though 

 he admits the overgrowth and immigration of cells, as well as 

 the persistence of living nuclei where the injury has been only 

 partial; all these contribute to the formation of the embryo, 

 which would be complete were its development not hindered by 

 the presence of the inert mass of yolk. 



The differences of interpretation put by Roux and Hertwig 

 on the same phenomena appear to be radical. A reconciliation 

 is, however, possible, for Morgan has observed that the whole or 

 half development of the injured cell depends upon the position 

 it takes up. If the original position with the black pole 



