112 DEVELOPMENT OF THE FROG'S EGG [Cm. XI 



shall be then in a position to understand more fully the results 

 of the experiments on the frog's egg. 



Hertwig ('93, b) was the first to repeat Roux's experiment, 

 but reached results diametrically opposed to those of Roux. 

 At the two-cell stage, one of the blastomeres was stuck with 

 a hot needle, 1 but unfortunately a detailed description of the 

 method employed is not given by Hertwig. After the opera- 

 tion 2 the egg so turns itself that the uninjured part rotates 

 upward, while the injured half is below. This is owing, Hert- 

 wig says, to the development of a blastula and gastrula cav- 

 ity, within the uninjured and segmented half. The cleavage- 

 stages of the egg are not described! Sections of the blastula 

 stage show that in the cellulated half a segmentation-cavity, 

 having a thin roof, has appeared. This cavity lies, in the 

 present case, in the centre of the developing half. In other 

 embryos, the cavity may lie excentrically, and in some cases a 

 part of the floor of the cavity may be bounded by the yolk-substance 

 of the undeveloped half. Hertwig interprets these results to 

 mean that when one of the first blastomeres is injured, the 

 method of development of the other blastomere is very much 

 altered. The injured half lying in contact with the active 

 half plays only a passive role in the further development. 



The injured blastomere is closely applied to the developing 

 half, and in places passes continuously into the latter. Hertwig 

 thinks that the yolk of the injured blastomere exerts on the 

 developing half an influence similar to that which the food- 

 yolk of meroblastic eggs exerts on the protoplasmic portion 

 that forms the embryo. This injured yolk-material comes to 

 lie in the ventral and posterior portion of the embryo. 



Hertwig ventures further to prophesy that if the injured 

 yolk-mass had been taken altogether out of the egg-coat (i.e. 

 from its contact with the living half), then there would be 

 formed a normal embryo without defect and like the normal 

 embryo in every respect except its smaller size. 



It is of importance to note that Hertwig describes other 



1 In a few cases a galvanic stream was used to kill the blastomere. 



2 How soon after is not stated. 



