44 SCIENCE PROGRESS. 



of osculum-bearing persons. No simpler type of budding 

 can be conceived than that exhibited by the Asconidse. In 

 the Hydrozoa the phenomena are practically identical. An 

 attempt was made, it is true, by Albert Lang (i§), one of 

 Weismann's pupils, to derive the buds of Hydroidea from 

 the ectoderm alone ; but the recent researches of Seeliger 

 (28) have fully confirmed our earlier ideas upon the 

 subject. Budding in Hydrozoa involves the participation 

 of both germ-layers. The buds of the Diploblastica are 

 two-layered. 



It is not an object of the present article to make a general 

 survey of budding in the Triploblastica. So far as the 

 Tunicata are concerned, no fact has been more clearly 

 established than this, — that in every one of the different 

 types of budding found within the group the two primary 

 germ-layers of the parent directly participate ; the ectoderm 

 and endoderm of the buds are direct prolongations of the 

 corresponding layers of the parent. The only instance that 

 is likely to be cited as an exception to this statement is that 

 of the Botryllidae, where the inner layer of the bud is a 

 prolongation of the atrial epithelium of the parent. Hjort 

 (9, 10) maintains that the inner layer is consequently of 

 ectodermal origin, a point which, if true, would render 

 utterly useless any attempt to reconcile the phenomena of 

 blastogenesis in Tunicata with the theory of differentiated 

 germinal layers, since the whole of the pharynx and 

 digestive tract of the bud is derived from the internal layer. 

 Hjort's conclusion, however, is founded entirely on the 

 analogy of those Ascidians in which the peribranchial 

 chambers have been shown to be derived from ectodermal 

 involutions ; whereas it has been shown by Pizon (20) 

 that in the embryos of Botryllidse the peribranchial 

 chambers are exclusively derived from endodermal diver- 

 ticula. It is the old story of the subtlety of nature 

 exceeding the subtlety of human- wit. 



But the buds of Tunicata are not built up exclusively 

 from the two primary layers. In all cases which have been 

 properly examined some derivatives of the mesoblastic 

 tissues of the parent are also included within the bud, 



