The Ascidian Half-Embryo. 51 



Although at that time reluctant to admit anywhere the occur- 

 rence of " partial " development, Driesch has since proved, in con- 

 nection with Morgan, the existence of a partial early development 

 in the ctenophore egg.^ And in a recent paper by the writer f 

 it has been shown that the isolated blastomere of the snail pos- 

 sesses the power of forming only a corresponding portion of an 

 embryo. In a later paper, Driesch,J developing an idea suggested 

 by Prof. E. B. Wilson and myself (loc. cit.), recognized the ex- 

 istence of a series among animal eggs, from the nearly isotropic 

 eggs of the medusa, Amphioxus, fish, sea-urchin, etc., at one ex- 

 treme, to forms such as the frog and ctenophore, and finally to 

 the snail, at the other extreme, where the blastomere possesses 

 such an organization that but a part of an embryo can be formed 

 and postgeneration cannot occur. 



The ascidian egg, however, remained unexplained by the contra- 

 dictory results of Chabry and Driesch. From this consideration 

 the author was led to an examination of the facts in another 

 ascidian. The results will, it is hoped, clear up the confusion to 

 some extent, and will show how far the development is a " partial " 

 one and in what respects it is " total." 



The experiments were performed during the past summer at the 

 Marine Biological Laboratory, Wood's Holl, upon the eggs of 

 Molgula manhattensis, which grows very abundantly upon the 

 piles and wharves at New Bedford, Mass. Artificial cross-fertil- 

 ization was resorted to, and the eggs at the desired stage were 

 spurted in a watch-glass by means of a fine spiral pipette. § Those 

 eggs presenting isolations were placed separately in watch-glasses, 

 and camera drawings of successive stages were made at intervals, 

 using a Zeiss oc. 4, and obj. C. 



As to nomenclature, the system proposed by Kofoid [] and ap- 



* Driesch, H., and Morgan T. H. Von der Eutwickelung einzelner Cteno- 

 phorenblastomeren. Archiv fur Entwick. der Organismen. II. 2. 1895. 



t Crampton, H. E., Jr. Experimental Studies on Gasteropod Development, 

 with an appendix on Cleavage and Mosaic Work, by E. B. Wilson. Archiv 

 fur Entwick. der Organistnen. III. 1. 1896. 



% Driesch, H. Betrachtung uber die Organisation des Eies und ihre Genese. 

 Archiv fur Entwick. der Organismen. IV. 1. 1896. 



I As previously described in connection with the gasteropod experiments. 



|| Kofoid, C. On some laws of Cleavage in Simax. Proc. Amer. Acad. 

 Arts and Sciences. Vol. XXIX. 1894. 



