60 SCIENCE PROGRESS. 



are examples of transverse fission as distinct from budding. 

 But the former is simply an instance of precocious budding, 

 as Korschelt and Heider (15) have shown. And, in the case 

 of the Polyclinidse, the post-abdomen, although it contains the 

 heart and genital glands, seems — as Giard and Lahille have 

 remarked — to be merely a portion of the stolon into which 

 these organs have been secondarily prolonged. The so- 

 called transverse fission of the Polyclinidse is therefore only 

 a special mode of stolonial budding, and may be compared 

 with the allied processes exhibited by such forms as 

 Clavelina and Distaplia. In the former genus the buds 

 remain structurally connected with the parent throughout 

 life ; but in Distaplia, a form which closely resembles 

 Clavelina in structure, the buds produced by the stolon are 

 detached early, and move to the surface of the colony in 

 just the same way as do the post-abdominal segments in 

 Amarcecium and other Polyclinidae. 



In an earlier part of this paper I have drawn attention 

 to the fact that budding in Tunicata takes place upon one 

 or other of two plans, which were termed u differentiate " 

 and " undifferentiate " ; and that the former type is especially 

 characteristic of the pelagic forms (Thaliacea), while the 

 latter type is predominant in the fixed forms (Ascidiacea). 

 The distinction between these two types is not, however, 

 sharply marked ; and the terms have been suggested for 

 descriptive rather than for classificatory purposes. 



A comprehensive survey of the phenomena has led me 

 to the conclusion that the true grounds of classification must 

 be sought in the various modes by which the endodermal 

 elements of the buds originate. It is now well known that 

 in fixed Ascidians the endoderm of the bud is derived in 

 most cases from a pair of diverticula from the posterior 

 region of the pharynx of the parent — the epicardial tubes of 

 Van Beneden and Julin. These peculiar organs are without 

 any homologues in Appendicularians or the higher Chordata, 

 and have clearly been developed within the group in relation 

 with the evolution of buddino. Their occasional connection 

 with the heart {e.g., Clavelina) must be regarded as a 

 phenomenon of secondary importance. In many Ascidians 



