II. — The Ascidian Half-Embryo. 



BY HENRY E. CRAMPTON, JR. 

 Read March 8, 1897. 



The development of isolated blastomeres of the ascidian egg- 

 has afforded a subject of considerable discussion on the part of 

 many theoretical embryologists. Chabry* approached the subject 

 from the experimental side, and, from the results of his many de- 

 tailed observations and experiments, was led to the conclusion 

 that one of the isolated blastomeres of the two-celled stage pro- 

 duced a strict half-embryo. As it was well known that .the first 

 cleavage-plane divided the egg into right and left halves, this con- 

 clusion seemed altogether probable and of considerable interest. 



A number of writers, however, among them Hertwig,f Driesch,J 

 Weismann,§ Barfurth|| and Roux,^[ were led, on the grounds of 

 Chabry's results, to opinions more or less at variance with his. 

 Barfurth considered Chabry to be in greater part correct. Roux 

 and Weismann believed that during the later development the 

 missing part was supplied by the other cells through " postgene- 

 ration," Hertwig states that, in his opinion, Chabry was in er- 

 ror ; and Driesch also argued that a t} T pical total development oc- 

 curred. Finally, Driesch** in 1893 repeated Chabry's experiments, 

 upon the eggs of Phallusia mammillata, and by the results 

 wholly confirmed the theoretical conclusions of his previous paper. 



* Chabry L. Contribution a l'embryologie normale et teratologique des 

 ascidies simples. Journ. de l'anat. et de la phys. XXIII. 1887. 



t Hertwig, E. Urmund und Spina bifida. Arch. f. mikr. Anat. XXXIX. 

 1892. 



% Driesch, H. Der Werth der beiden ersten Furchungszellen in der Echino- 

 dernientwickelung. Zeit. f. wiss. Zool. LIII. 



\ Weismann, A. Das Keimplasma. 1882. 



|| Barfurth, D. Halbbildung oder Ganzbildung von halber Grosse. Anat. 

 Anz. VIII. 1893. 



If Eoux, W. Uber des entwickelungsmechanische Verniogen jeder der bei- 

 den ersten Furchungszellen des Eies. Verhandl. d. Anat. Ges. Wien. 1892. 



** Driesch, H. Von der Entwickelung einzelner Ascidienblastomeren. 

 Archiv fur Entwick. der Organismen. I. 3. 1895. 



