REGENERATION IN EGG AND EMBRYO 219 



embryo. If one half had been removed, we can foresee that its 

 absence might lead to other complications that would affect the 

 result. 



The most important outcome of this experiment is, I think, to show 

 that a half-structure may develop by itself, i.e. that there is a certain 

 amount of independent power of development in the parts of the egg. 



Roux also tried to show that if, after the second cleavage has been 

 completed, the two blastomeres that lie on opposite sides of the first 

 cleavage plane are killed by a hot needle, the remaining two produce 

 either an anterior or a posterior half of an embryo. An embryo 

 derived from the two "anterior" blastomeres is represented in Fig. 

 61, D. The anterior half of the body is present. Posteriorly the 

 half-embryo abuts against the injured half. It is possible, I think, 

 that this embryo may represent the anterior half of a whole embryo 

 of half size that has been prevented from closing in posteriorly by the 

 mass of injured material of the undeveloped blastomere. Roux did 

 not determine positively whether the two "posterior" blastomeres 

 could give rise to posterior half-embryos ; one embryo in his opinion 

 appeared to bear out this interpretation. This part of Roux's work 

 is, it seems to me, not so satisfactory as the part dealing with the 

 first two blastomeres, and we may leave it, for the present, out of the 

 discussion, and consider only the result of the first experiment, in 

 which one of the first two blastomeres was injured. Since the prob- 

 lems involved in the two cases are essentially the same, nothing will 

 be lost by dealing with the first case alone. 



The uninjured blastomere first gives rise to a half-embryo. After 

 this has been accomplished, other changes take place that " reorgan- 

 ize," according to Roux, the material of the injured half in such a way 

 that the missing half of the embryo is formed by a process that Roux 

 calls "post-generation." This process can be studied only by means 

 of sectioning the embryos, and since the eggs may be injured to a 

 varying extent, there must be some uncertainty in making out the 

 sequence of events. It is found that the yolk of the injured blasto- 

 mere is vacuolated in places, and that the protoplasm in the path of 

 the needle has been killed (Fig. 61, A). Irregular pieces of chromatin 

 are found in the protoplasm, which seem to come from an irregular 

 breaking up of the nucleus. 



The changes that lead to the reorganization of the injured half 

 may take place at different times in different eggs. Roux describes 

 three kinds of reorganization phenomena. The first includes the 

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

 finely granular protoplasm, appear in the protoplasm of the injured 

 blastomere. These nuclei arise from two sources : in part from the 

 scattered chromatin of the injured blastomere itself, and in part from 



