64 SCIENCE PROGRESS. 



(ii.) Large. Pyrosomidse, Salpidae. 

 Group 5. Buds median and posterior. 



(i.) Permanently connected with parent. Clavelina, 

 Peropkora. 



(ii.) Detached from parent. 



(a) Formed from simple stolon. Distaplia. 

 (6) Formed from post-abdomen. Polyclinidae. 

 4. The Origin of Budding. — If I have been at all suc- 

 cessful in my analysis of the various types of budding in 

 Tunicata, and in my treatment of the evolution of the 

 epicardial tubes, it must be clear to the readers of this 

 article that the existing types of budding represent merely 

 the final stages of a long process of evolutionary change. 

 Upon first considerations one is tempted to regard the 

 simple process of budding, which I have called undifferen- 

 tiate, as more primitive than the differentiate. In the 

 undifferentiate buds of Clavelina and other forms the whole 

 process of organogeny is strikingly similar to the growth 

 of an embryo from the gastrula stage ; and this analogy 

 inclines one at first to believe that the simple structure of 

 the undifferentiate bud has the same primitive significance 

 as that of the gastrula. The phylogeny of budding, how- 

 ever, cannot be determined by the study of the bud alone ; 

 it is necessary to consider also the associated changes 

 which have taken place in the structure of the parent 

 form. 



Attention to this consideration reveals the fact that the 

 primitive character which one is inclined to ascribe to the 

 triploblastic bud of Clavelina, on account of its simple 

 structure, is not borne out by the character of the associated 

 structures of the parent form. We have already seen that 

 the epicardial tube of Clavelina is not a primitive structure, 

 but results from the fusion of a pair of tubes ; that median 

 budding was preceded phylogenetically by lateral budding ; 

 and that the lateral buds of Botryllus have a more differen- 

 tiate character than the median stolonial buds of Clavelina. 

 Another point in this phylogenetic history must be 

 mentioned. In the larva of Distaplia two epicardial tubes 

 are present, but one only is functional. In Botryllus, also, 



