46 SCIENCE PROGRESS. 



case, owing to the addition of another element or organ- 

 process to the three layers typically present. In Salpa also 

 the buds are four-layered from the beginning. The peri- 

 branchial sacs of the stolon, unlike those of the primary 

 ascidiozooids of Pyrosoma, develop quite independeptly of 

 the parental organ ; but a genital rod derived from the 

 parent is present throughout the stolon and supplies a fourth 

 element to each of the buds. It is true that this particular 

 element takes little or no part in the formation of the 

 somatic tissues of the bud ; but, as the presence of generative 

 organs is a characteristic feature of the asexual generation 

 of Salps, it is important to observe that these organs are not 

 secondarily developed from one of the primary layers of the 

 bud but are directly derived from the preceding genera- 

 tion. This transference of reproductive elements from one 

 oeneration, either sexual or asexual, to another, which is 

 always produced by budding, is a striking feature of Tunicate 

 blastogenesis. It has been thoroughly established in several 

 forms, e.g., Salpa (Brooks), the secondary ascidiozooids of 

 Pyrosoma (Joliet, Seeliger), Botryllus (Pizon) ; and it possibly 

 occurs also in other forms such as Clavelina and Perophora, 

 although the similarity between the young germ-cells and 

 the general mesenchyme of these types has hitherto pre- 

 vented any exact demonstration of the point. 



The constitution of the bud would appear to be still more 

 complicated in the genus Doliolwm. Both Grobben (8b) 

 and Uljanin (3 1 ) state that in this form the ectoderm envelope 

 encloses as many as five separate kinds of embryonic 

 elements, of which some are unpaired and some paired. 

 The origin and fate of these constituents, however, is still 

 so imperfectly ascertained that it seems to me to be altogether 

 impossible to make any detailed use of the current statements 

 upon the budding of Doliolum. It seems to be fairly well 

 established, however, that the stolon (and buds) of Doliolum 

 contain two pairs of prolongations from the pharynx and 

 cloaca of the parent, and an unpaired outgrowth of the 

 parental mesoderm. The paired outgrowths of the pharynx 

 I regard as homologous with the epicardial tubes of 

 Ascidians, and the diverticula from the cloaca readily lend 



