312 A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 



Article X. 



On the Development of the Ascidians. By A. Krohn. 



[From Miiller's Archiv, 1852-1853.] 



In setting forth the results of my observations on the develop- 

 ment of the Ascidians, it is nowise my intention to lay before 

 the reader a connected account of all the appearances which 

 they present. Considering the earlier works in this department, 

 I shall endeavour to exhibit only the most important steps, and 

 in connexion therewith, as occasion may serve, to consider some- 

 times one, sometimes another organ, whose mode of development 

 may possess a special interest. I will premise here that the 

 facts I am about to communicate all relate to the development 

 of the Phallusia mammillata of Cuvier, and have been obtained 

 by the employment of artificial fecundation *. By this method 

 I have succeeded in following out their progress step by step 

 for three months, until the young Ascidians had attained the 

 size of a line or more. I commence with the unfecundated 

 ovum, inasmuch as its structure is distinguished by many im- 

 portant peculiarities. 



1. Unfecundated ovum. — The fully formed ova which fill the 

 oviduct consist, most externally, of a villous investment, within 

 which is the proper vitellary membrane. Under the vitellary 

 membrane there lies a glassy layer, in which roundish green 

 structures of a peculiar kind are imbedded, and which surrounds 

 the colourless yelk. The germinal vesicle and spot, both of 

 which are still readily distinguishable in the ovarian ova, are 

 already wanting f. 



The investing membrane is thin, and beset superficially by 



* The first successfully conducted attempts to fecundate the ova of the 

 PhallusicB were made by Von Baer. 



t The same structure is exhibited by the unfecundated ova of the Phal- 

 lusice in general, as well as by those of the genus ClaveUna. 



