PHALLUSIA MAMILLATA 403 



the egg. In Phallusia the | larvae which develop from one of 

 the first two blastomeres may appear superficially complete, but 

 sections show that they always lack the organs characteristic of 

 the missing half. Anterior or posterior t larvae, and all I larvae 

 are extremely atypical and incomplete. The organs which 

 typically arise along the median plane and from both right and 

 left halves may be present in the larvae derived from either half 

 (e. g. chorda, enteron, neural plate), but in no case does an organ 

 or tissue arise from cells which typically would have given rise 

 to a different kind of organ or tissue. Thus cells containing the 

 substance of the mesodermal crescent do not give rise to any 

 thing other than muscle or mesenchyme, and in this sense the 

 substances of the egg of Phallusia, which are already localized 

 at the time of the first and second cleavages, are 'organ forming.' 

 In these respects Phallusia is not exceptional among ascidians. 

 There is good reason to believe that ascidians as a class are charac- 

 terized by the possession of eggs having a relatively high degree 

 of differentiation and a low degree of regulation. 



REFERENCES 



Chabry, L. 1887 Contribution a I'embryologie normal et teratologique des 

 Ascidies simples. Jour. Anat. et Physiol. Tome, 23. 



CoNKLiN, E. G. 1905 The organization and cell-lineage of the Ascidian egg. 

 Jour. Acad. Nat. Sci. Philadelphia, vol. 12. 



1905 Mosaic development in Ascidian eggs. Jour. Exp. Zool. vol. 2. 



1906 Does half of an Ascidian egg give rise to a whole larva? Arch, 

 f. Entwicklungsmech, vol. 21. 



Driesch, H. 1895 Von der Entwicklung einzelner Ascidienblastomeren. Arch, 

 f. Entwicklungsmech, Bd. 1. 



1905 Die Entwicklungsphysiologie von 1902 bis 1905. Ergebnisse der 

 Anat. und Entwickungsgeschichte, Bd. 14. 



