REGENERATION IN EGG AND EMBRYO 22$ 



ing that condition (Fig. 62, D, E, F, G, H\ It is shorter than the 

 normal embryo, and its posterior end is incomplete. When these 

 embryos are cut into sections, it is found that the part that has 

 developed corresponds to the dorsal part of a normal embryo, but 

 the ventral part is continuous with the yolk substance of the injured 

 half (Fig. 62, B, C, J, K), Hertwig interprets these embryos as 

 forms in which the yolk portion of the developing half, together with 

 the whole of the injured blastomere, represents a yolk mass that has 

 not yet been enclosed by the margin of the developing part. 



In nearly all the embryos that Hertwig has described, the medul- 

 lary folds appear eccentrically on the developing half (Fig. 62, D, F, 

 K\ and in some cases they may lie so far to one side that they are 

 situated almost at the edge ; and the less development of one of the 

 folds makes the embryo appear almost like the hemi-embryos obtained 

 by Roux. In fact, one embryo seems to have been a true hemi- 

 embryo. 



Hertwig attributes the eccentric position of the embryo to the 

 eccentric position of the blastopore of an earlier stage, but he does not 

 attempt to account for the eccentricity of the latter. 



It is significant in this connection to find that Hertwig obtained 

 other embryos that show a condition of " spina bifida." In these 

 there is an exposure of yolk in the mid-dorsal line between the halves 

 of the medullary folds. Still other embryos in the same series of 

 experiments were only slightly injured, and developed nearly nor- 

 mally. In these cases, Hertwig thinks, the blastomere that was 

 stuck had been only slightly injured, and had partly developed. I 

 have also often observed in this experiment that the injured blasto- 

 mere may segment and add cells to the developing half, but in such 

 cases the development of the injured half may be less regular than is 

 that of the uninjured half. It seems to me not improbable that in sev- 

 eral of the embryos described by Hertwig both blastomeres have taken 

 part in the development. The main points of difference between 

 the results of Roux and of Hertwig cannot, however, be explained 

 in this way, and the explanation is to be found in another direction. 



Hertwig emphasizes the view that the injured blastomere is not 

 dead, but exerts an influence upon the other half an influence of 

 the same kind as that which the yolk of a meroblastic egg has on the 

 protoplasmic portion of the egg from which the embryo arises. He 

 ventured to prophesy that if the injured yolk mass could be entirely 

 removed, the uninjured blastomere would produce a normal embryo 

 without defect, and one like the normal embryo in every respect 

 except in size. 1 



1 The development of isolated blastomeres of the ctenophore egg shows that this need 

 not be the case. 



