308 LECTURES ON BIOLOGY 



and thinks it impossible to escape the conclusion that the rib 

 ' determinants ' are only distributed to certain daughter-cells. 

 But we cannot accept this conclusion, for the ova of the Cteno- 

 phores are distinguished by a highly differentiated structure, and 

 this may well be the reason why after an experimental opera- 

 tion regulative transformations no longer take place, but part- 

 formations develop instead. Again, it is observed in the ova of 

 Echinoderms that the isolated blastomeres divide at first 

 exactly as if they were still connected with the others ; in other 

 words, they produce at first part-formations. It might, there- 

 fore, seem as if here, too, heterokinesis had taken place, but after 

 some time we observe that regulative transformation-processes 

 set in and that after all normal larvae develop from the part- 

 formations. 



This opinion is supported by experiments made with ova 

 of amphibians by Eoux, Hertwig, Morgan, and Herlitzka. If 

 Roux killed one of the first pair of segmentation-globules of a 

 frog-ovum by puncturing it with a hot needle, the sound half 

 continued to develop as if no operation had taken place, i.e., 

 segmentation stages were developed which corresponded to the 

 lateral half of a corresponding normal developmental stage. It 

 even happened sometimes that such section of the ovum developed 

 into a complete lateral half-embryo (Hemi-embryo lateralis). By 

 subsequent regeneration, the so-called post-generation, these 

 half-embryos were sometimes afterwards able to supplement the 

 deficient body-half. 



Hertwig repeated Roux's experiments with frog eggs, but 

 reached entirely different results. According to him the sur- 

 viving blastomeres developed at once complete embryos, which 

 exhibited but slight defects, and even then only in unimportant 

 body parts. 



Morgan succeeded in explaining the apparently contradictory 

 results obtained by these two investigators, and demonstrated 

 that the position of the uninjured segmentation-cell after the 

 operation greatly influences its further fate. Even with the 

 naked eye w r e are able to recognize in the frog-egg a dark side 

 rich in plasm, and a light side rich in yolk; they are usually 

 described as the animal and vegetal poles. During the normal 

 course of development the light-coloured vegetal pole, being 



