384 PROGRESS OF SCIENCE IN THE CENTURY. 



intact blastomere be inverted, then it may develop 

 into a half -embryo (3 cases) or into an entire dwari 

 (9 cases). 



" Morgan therefore concluded that the production 

 of whole embryos by the inverted blastomeres was, 

 in part at least, due to a rearrangement or rotation 

 of the egg-materials under the influence of gravity, 

 the blastomeres thus returning as it were, to a state of 

 equilibrium like that of an entire ovum." * 



(c) Isolation-Experiments. Professor C. Chun 

 observed in 1877 that when the two first segmenta- 

 tion-cells of a ctenophore ovum were shaken apart, 

 each formed a half-larva, with four instead of eight 

 ciliated ridges and meridional vessels, with one ten- 

 tacle instead of two. The half-larva3 actually be- 

 came sexual, and by a process of budding, the missing 

 half was eventually formed. The observer also 

 added the interesting note that united twin cteno- 

 phore-larvse were most abundant after stormy days, 

 probably resulting from the incomplete separation 

 of the first two blastomeres and their independent 

 development. 



The importance of Chun's hint was recognised 

 by Driesch who was the first to develop the method 

 of isolating segmentation-cells by shaking. The 

 device has been resorted to in many cases, with 

 ascidians and sea-urchins in particular. As a par- 

 ticularly fine piece of work, we may refer to Prof. 

 E. B. Wilson's experiments on the eggs of the lance- 

 let (Amphioxus) .f 



By shaking the water in which the two-celled 

 stages floated, Wilson separated the two cells, and 



* B. B. Wilson, The Cell in Development and Inheritance, 

 2nd ed., 1900, p. 422. 

 t Journal of Morphology, VIII. (1893), pp. 579-638, 10 



pis. 



