5 o SCIENCE PROGRESS. 



among the fixed Ascidians of a phenomenon of histo- 

 genesis which reaches its most striking development in the 

 Thaliacea (especially Pyrosoma and Salpd). 



At the present moment I do not wish to prejudice the 

 question as to which of these types of budding is the more 

 primitive. One is always inclined to regard the simpler of 

 any two phenomena of the same kind as the more primitive, 

 and on this account the " undifferentiate " type, character- 

 istic of the Ascidiacea, at first sight naturally appears to be 

 the primitive form of asexual reproduction, of which the 

 "differentiate" type is a later and secondary elaboration. The 

 question cannot be adequately discussed, however, before a 

 fuller survey of the phenomena has been made ; and the 

 treatment of it will therefore be postponed to a later section 

 of this article. 



2. The Origin of Special Organs in the Btid. — Remarks 

 have been made in the preceding section which indicate that 

 the same or homologous organs may develop in the buds of 

 different types by processes which are not always identical, 

 and that these may also be dissimilar from the processes 

 typical of embryonic development. The literature upon the 

 subject is full of such instances ; and even if only half of these 

 asserted instances are actually true, development by budding 

 must be regarded as capricious and irregular to an extra- 

 ordinary degree. The writer who endeavours to trace the 

 order underlying a mass of conflicting statements about 

 phenomena which in themselves are remarkably varied, has 

 to avoid a double danger, — the risk of wasting his efforts by 

 an excess of timidity in dealing with the materials before 

 him, and the tendency to constrain the facts into a system 

 more rigid and narrow than nature has prescribed. In the 

 present case I scarcely hope to be successful in avoiding both 

 these dangers ; but it certainly seems most desirable that 

 some attempt should be made to estimate the value of the 

 conflicting statements which exist, and to systematise the 

 facts which appear to have been most satisfactorily ascer- 

 tained. 



The development of the peribranchial sacs has perhaps 

 furnished the chief difficulty in attempts to apply the germ- 



