CHAPTER XI 



THE EFFECT OF INJURING ONE OF THE FIRST TWO 

 BLASTOMERES 



WE have seen that the plane of first cleavage corresponds as 

 a rule with the median plane of the future embryo, so that one 

 of the first two blastomeres gives rise to the cells that form one 

 side of the body of the embryo, and the other blastomere pro- 

 duces the cells of the other side. It would seem then that 

 even at the two-cell stage the axes of the future embryo are 

 definitely laid down. But the most fundamental question re- 

 mains unanswered; viz., has the egg after its first cleavage 

 divided its material into qualitatively different parts (i.e. 

 has the material of the right side of the body been separated 

 qualitatively from that of the left side), or are the first-formed 

 blastomeres still undifferentiated, and their subsequent fate 

 dependent on the relative position they bear to each other as 

 a part of a whole f 



Roux tried to answer this question by the following ingenious 

 experiment. 



ROUX'S EXPERIMENT OF " KILLING " ONE OF THE FlRST 



Two BLASTOMERES 



As soon as the first furrow had passed through the egg, one 

 of the resulting blastomeres was pierced with a hot needle. 

 In order to carry out the experiment successfully, certain pre- 

 cautions must be taken. The eggs as soon as removed from 

 the uterus are scattered over a glass plate (under water) so 

 that they lie singly. Then water containing spermatozoa is 

 added. After ten minutes this water, clouded by the sperma- 

 tozoa, is poured off and fresh water is added. When the first 

 furrow in the eggs appears, the water is again poured off. 

 Each egg is held by a pair of forceps and then pierced by a 



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