136 THE GERM-PLASM 



its prescribed course of development apart from the rest of the 

 embryonic cells, — or whether the various cells of the embryo 

 become differentiated by their mutual interaction : or, in other 

 words, whether a determinating influence is to a certain extent 

 exerted by t)ie whole on its parts and thus prescribes the fate of 

 the various cells. 



The experimental proof of the self-differentiation or predis- 

 position of the individual cells was, I believe, furnished by 

 Roux,* whose ingenious experiments are always accompanied 

 by keen deductions. Roux destroyed a single segmentation- 

 cell in each of a series of frogs' eggs by means of a hot needle, 

 and then observed that eggs treated in this manner developed 

 into ' half or three-quarter embryos,' that part being absent 

 which corresponded to the cell thus destroyed. When one 

 of the first two segmentation-cells was demolished, half of the 

 embryo was formed, and this corresponded either to a lateral 

 or to the anterior or posterior half, according to whether the 

 first segmentation had resulted in a division of the hereditary 

 substance into portions belonging to the right and left, or to the 

 anterior and posterior halves. The process of segmentation in 

 the frog is known to vary in this respect. When one of the first 

 four segmentation-cells was destroyed, three-quarters of the 

 embryo was formed. 



These experiments must be regarded as affording a proof of 

 the self-differentiation of the cells. Observations have since been 

 made which seem to contradict this deduction ; and although 

 these are still incomplete, and can only be regarded as the 

 preliminaries to more detailed investigations, they must not be 

 passed over in silence, especially as I am convinced that they 

 do not really contradict the hypothesis of the self-determination 

 of the cells. 



Chabry's f experiments on the eggs of Ascidians must be 

 mentioned first. By means of a special apparatus, he destroyed 

 one of the first two segmentation-cells, and then observed that 

 the remaining cell continued to develop, and eventually gave 

 rise, not indeed to half an embryo, but to an entire one of 

 half the normal size. Such embryos were certainly not quite 



* Wilhelm Roux, ' Beitrage zur Entwicklungsmechanik des Embryo,' 

 (V.) Virch. Arch. Bd. 94. 



t L. Chabry, ' Embryologie normale et t6ratologique des Ascidies,' Paris, 

 1887. 



