BUDDING IN TUNIC AT A. 45 



between the endodermal vesicle and the ectodermal 

 envelope. These derivatives generally take the form 

 of wandering mesenchyme cells, and are swept along 

 between the two primary layers by the force of the 

 maternal blood current. Many of them become applied to 

 the epithelial walls of the developing bud, and build up its 

 musculature and other skeletotrophic tissues. 



The Tunicate bud in its typical form may accordingly be 

 said to consist of two epithelial vesicles one inside the other, 

 and of a certain quantity of mesenchymatous tissue between 

 the vesicles. The two vesicles are derived by evagination 

 from the two primary layers of the parent, and are at first 

 in direct continuity with them ; the mesenchyme is also 

 derived from the corresponding tissue of the parent. Even 

 in the minute ventral buds of Distaplia Salensky (25) has 

 recently shown that several parental mesenchyme cells are 

 included which furnish the rudiment of the future mesodermal 

 tissues. On the other hand the statements of Korotneff 

 (12) upon the triploblastic nature of the buds of Dolckinia 

 must be accepted very cautiously ; for not only are we 

 without any actual evidence as to the origin of the three 

 elements he describes, but their later fate renders the nature 

 which Korotneff has ascribed to them extremely doubtful. 



So far as the evidence goes, therefore, the Tunicate bud 

 is composed of derivatives of the three germ-layers ; and in 

 this group of the Triploblastica at any rate the process of 

 blastogenesis is in complete harmony with the germ-layer 

 theory so far as the constitution of the buds is concerned. 



Nevertheless, although I am not aware that any 

 investigator has satisfactorily established any instance of a 

 Tunicate bud possessing an organisation simpler than this, 

 it is an interesting fact that in certain cases the bud exhibits 

 a greater complexity. In the four primary ascidiozooids of 

 Pyrosoma, for example, not only are the general ectoderm, 

 endoderm, and mesoderm of the parent cyathozooid pro- 

 longed to form the corresponding tissues of the buds, but a 

 pair of special prolongations of the peribranchial sacs is also 

 contained in the developing stolon. Instead of being 

 triploblastic, the buds become actually four-layered in this 



