382 PROGRESS OF SCIENCE IN THE CENTURY. 



in it the material for forming the left half of tho 

 body. 



When Roux punctured one of the first two segmen- 

 tation-cells (or blastomeres) with a hot needle or 

 otherwise, he found that the intact other cell devel- 

 oped into a typical 7mZ/-morula, or /&aZ/-gastrula, or 

 7iaZ/-embryo, according to the success in survival. 

 Thus, there might be in the embryo, half of the 

 normal cerebrum, one ear-sac, a one-sided gut, a 

 single row of protovertebrae, and so on. Thus it was 

 proved that one of the first two segmentation cells (or 

 blastomeres) may form half an embryo; it has the 

 requisite material and the requisite power of inde- 

 pendent development. This, and many similar ex- 

 periments, led Eoux to his theory that the early de- 

 velopment of the frog-ovum is like a kind of mosaic 

 work pieced together in independent parts. He sug- 

 gested that there were at least four independently-de- 

 veloping pieces. It should be noted, however, that 

 the half -embryo may eventually form a whole, either 

 with the aid of a re-vitalisation of the injured half- 

 egg which has been lying passive while the uninjured 

 half was developing, or even without any co-opera- 

 tion on the part of the injured half of the first 

 cleavage.* 



So far, there seemed to be a definite conclusion 

 reached by an investigator of the first rank, that the 

 puncturing of one of the first two cells into which 

 a frog's egg divides, has for its result that the intact 

 other cell forms a half -embryo, a one-sided em- 

 bryo, which by " post-regeneration " may become 

 eventually a whole. 



But in 1893, Professor Oscar Hertwig, whose con- 



* Virchow's Archiv f. Pathologic, CXIV. (1888). 



