332 TIIK BEGINNINGS OF CAUSAL MORPHOLOGY 



there are formed two embryos instead of one. In the same 

 year T. H. Morgan l repeated Roux's fundamental experiment 

 of destroying one of the two blastomeres, but inverted the 

 egg immediately after the operation a whole embryo of half 

 size resulted. A year or two later Herlitzka' 2 found that if 

 the first two blastomeres of the newt's egg were separated by 

 constriction, two normal embryos of rather more than half 

 normal size were formed. 



The main result of the first few years' work on the 

 development of isolated blastomeres was to show that the 

 mosaic theory was not strictly true, and that the hypothesis 

 of a qualitative division of the nucleus was on the whole 

 negatived by the facts. 



Evidence soon accumulated that the cytoplasm of the egg 

 stood for much in the differentiation of the embryo. A 

 number of years previously Chun had made the discovery 

 that single blastomeres of the Ctenophore egg, isolated at the 

 two-celled stage, gave half-embryos. This was in the main 

 confirmed by Driesch and Morgan in i896, 8 and they made 

 the further interesting discovery that the same defective 

 larvae could be obtained by removing from the unsegmented 

 egg a large amount of cytoplasm. Conclusive proof of the 

 importance of the cytoplasm was obtained soon after by 

 Crampton, 4 who removed the anucleate "yolk-lobe" from the 

 egg of the mollusc I/rdnassa at the two-celled stage, and 

 obtained larva;: which lacked a mesoblast. This result was 

 brilliantly confirmed and extended some years later by 

 I-",. 1). Wilson/' working on the egg of Dcntalinui. He found 

 that if the similar anucleate "polar lobe" of this form is 

 removed at the two-celled stage, deficient larvre arc formed, 

 in which the post - trochal region and the apical organ 

 are absent. lie further showed that in the unsegmented 

 but mature egg prelocalised cytoplasmic regions can be 

 distinguished, which later become separated from one 

 another through the segmentation of the egg. The seg- 

 mentation-cells into which these cytoplasmic substances 

 arc thus segregated show a marked specificity of develop- 



1 Ana/. Anz., x., i :' : Arch. /'. />//.- J/<v//., iv. 



.Irch.f. /i>;/. -.I/", r//., ii., i.SA ' Arch /. Knt.-.Wcch., iii., iSc;6. 

 iyV;-. /.i>i>/., i.. P 



