SOME POINTS IN THE BUD DEVELOPMENT OF A SIMPLE 

 ASCIDIAN, ECTEINASCIDIA TURBINATA HERDMANi 



By C. Dale Beers 



In the Clavellinidae the stolon consists of an outer ectodermal tube 

 enclosing an inner so-called epicardial tube, endodermal in origin, 

 the two tubes being separated by a blood sinus. The outer tube 

 arises from the ectodermal body wall of the larva, while the epi- 

 cardial tube is derived from the branchial sac of the larva. In 

 ClavelUna and Peropliora the walls of the inner tube are approxi- 

 mated so that it has the structure of a double-walled septum which 

 divides the outer tube into longitudinal halves, the septum stopping 

 short of the distal end of the stolon. The two lateral blood spaces 

 of the stolon remain in connection with the sinuses of the embryo- 

 zooid from which it has developed. In Ecteinascidia on the contrary, 

 the inner tube preserves its tubular shape. Transverse sections 

 show that the shape is irregular. In places the tube has no con- 

 tact with the surrounding ectodermal tube. In other places its wall is 

 at certain points in close contact with the ectodermal wall. Thus the 

 space separating the two tubes is not a simple undivided space but 

 consists rather of a set of sinuses continuous with one another. 



The bud primordium of Ecteinascidia is not unlike that of other 

 members of the family, and is made up of two vesicles, one within the 

 other, separated by a blood sinus. The outer or ectodermal vesicle 

 arises as an evagination from the outer tube of the stolon, the inner 

 vesicle as an evagination from the inner stolonic tube. It is the inner 

 or endodermal vesicle which is particularly concerned in the organ- 

 ogeny of the bud. From it develop not only the structures which are 

 derived from the endoderm in egg development, but others, such as 

 the nervous system and peribranchial sacs, which come from the ecto- 

 derm of the Q^^ embryo. It is this contrast in the embryogenic meth- 

 ods of larval development and bud development which demonstrates 

 that the idea of histogenetically specialized germ layers is inapplicable 

 to the Ascidians. 



^ The material used in this study was collected in the Bahama Islands by Professor H .V. 

 Wilson. Although old, its histological condition is good. Professor George Lefevre kindly 

 identified for me the form as Ecteinascidia turbinata. 



[92] 



