\S( IDIACEA — STOLONIC GEMMATION. 



l.M 



to resemble more and more the post-abdomen of the parent before 

 the commencement of asexual multiplication. The anterior part of 

 the body of the young individual lias shifted upward. The branchial 



and atrial apertures have broken through, and reproduction by means 

 of tranverse fission now takes place in the daughter-individuals that 

 have arisen as described above. 



We thus find, in this case, the first rudiments of the young animal 

 in the portion of the post-abdomen, which becomes separated by 

 transverse constriction, and which, through processes of regeneration, 

 is able to develop into a perfect new individual. For details of these 

 processes, see p. 470. 



Whether, in the segmentation of the post-abdomen in the Polyclinidae, we 

 actually have a more primitive form of asexual multiplication, from which 

 the stolonic gemmation of other Tunicates is to be derived, must still be 

 regarded as doubtful. It is also possible that the transverse division of the 

 Polyclinidae is to be derived from stolonic budding. A comparison with the 

 conditions of development of Pyrosom i shows that caution must be exercised 

 in such speculations. At first sight we should feel inclined to describe the 

 rise of the first four Ascidiozooids in the Pyrosoma embryo as transverse 

 fission (Fig. 193, p. 397), but more careful examination reveals the fact that 

 the later longitudinal axis of the Ascidiozooid is at right angles to that of the 

 proliferating stolon. We have, consequently, to regard the rise of these first 

 Ascidiozooids also as stolonic budding, which is not essentially distinguished 

 from the budding of the zooids that are produced later (see pp. 404 and 484). 



B. Stolonic Gemmation. 



The typical form of stolonic gemination is found in the so-called 

 social Ascidians, in Clavelina and Perophora. The single individuals 

 here send out a creeping proliferating stolon which branches re- 

 peatedly (Fig. 229), and at the end of which the buds appear as 

 club-shaped swellings. In structure the stolon closely resembles the 

 post-abdomen of the Polyclinidae. Here also we find, as has already 

 been shown, the blood-space of the stolon (primary body-cavity) 

 divided by the extension of the epicardial sac (stolonic septum, s) 

 into a dorsal and a ventral half, the blood circulating through these 

 in opposite directions. Since the partition-wall does not reach quite 

 to the distal end of the stolon (x) the two sinuses communicate at 

 this point, where also the stream of blood changes from the one 

 direction into the other. 



The buds here originally appear in the form of bilaminate vesicles 

 \kn). The outer layer of the vesicle, the ectoderm of the bud, is 

 continuous with the ectoderm of the stolon and of the parent- 



