110 DEVELOPMENT OF THE FROG'S EGG [Ch. XI 



of the half operated upon, and in the subsequent "postgenera- 

 tion" of the same. 



Sections of eggs that have been successfully operated upon 

 show the kind of change that has taken place in the injured 

 blastomere as a result of the operation. The yolk is found much 

 vacuolated in places, and the protoplasm in the immediate path 

 of the needle has been killed, and much changed. After a time 

 it is found that scattered nuclei or nuclear-like structures are 

 also present in the injured half (Fig. 35, A). These have come 

 from the regular or irregular division of the nucleus of the 

 blastomere that has not in most cases been killed by the hot 

 needle. The developed half is somewhat larger than the injured 

 blastomere, and a sharp line of demarcation is at first present 

 between the two halves. Even in the early stages of some eggs 

 changes are found to take place that precede the "reorganiza- 

 tion " of the injured half. Roux describes three sorts of re- 

 organization-phenomeyia. The first of these changes involves the 

 formation of cells in the injured half. Nuclei, surrounded by 

 a finely granular protoplasm, appear in the injured blastomere. 

 These nuclei seem to arise from two sources, — from the nucleus 

 of the injured blastomere, and from nuclei (or cells) of the 

 developing half that have transmigrated. Around the nuclei 

 the yolk breaks up into cells. This cellulation of the yolk may 

 take place at very different times. It may be absent in some 

 cases in a semigastrula and be present in other cases in a semi- 

 morula or semiblastula. The cellulation of the injured half 

 begins always near the developing half, and extends thence 

 outward. The cells of the injured half are of various sizes, 

 but generally larger than the cells of the uninjured half. 

 N The cellulation of the yolk takes place only in the unchanged 



non-vacuolated parts. Where the yolk has been much changed, 

 it is worked over by another method, i.e. by the second method 

 of reorganization. These parts are revived or reorganized by 

 the nuclei or the cells that have now appeared in the injured 

 half. Such parts are either actually devoured by wandering 

 cells or slowly changed under the influence of neighboring cells 

 or nuclei so that they become a part of these cells. 



In addition to the two preceding modes, a third method of 

 reorganization takes place. When the yolk has been much 



