264 SOME RECENT 



and perfecting of these parts, much as the flower is 

 formed by the expansion of parts already present in 

 the bud. This doctrine of Preformation, in its 

 original form, was overthrown in 1759 by Wolff, 

 who substituted for it the doctrine of Epigenesis />., 

 that there is no trace of the embryo or of any of its 

 parts in the egg, and that the formation of the 

 embryo is an entirely new process. It is not 

 a little curious to find the older doctrine, which 

 embryologists thought disposed of for ever, coming 

 up again in somewhat modified form, as a result of 

 more recent investigations. 



Almost simultaneously with Chabry, Roux in- 

 vestigated in similar manner, but in more detail, the 

 results of localised injuries to frog embryos at early 

 stages of development. His method consisted in 

 destroying one or more of the cells of a segmenting 

 egg by puncturing them with a hot needle. He 

 took a frog's egg at the completion of the first cleft 

 i.e., an egg which had just divided into the first two 

 cells or blastomeres and destroyed one of the two 

 cells. The surviving cell developed into a half- 

 embryo, from which by a process of regeneration 

 the missing half was gradually formed, a whole 

 embryo ultimately resulting. A more satisfactory 

 method of experimenting, inasmuch as it does not 

 involve the destruction of any of the cells, consists 

 in shaking apart the blastomeres at an early stage, 

 and then following their subsequent fate. This was 

 first done, in 1877, by Chun, and has since been 

 repeated by other observers. Chun experimented 

 with the eggs of Ctenophora, and found that if the 



