PYROSOMA FURTHER DEVELOPMENT OF THE BUDS. 489 



the enteric rudiment still connected with that of the next bud 

 {Fig. 2otf, ). The distal individual of the stolon is always the most 

 developed (cf. also Fig. 267). When five individuals have become 

 marked oft' on the stolon through the appearance of constrictions, 

 the distal individuals always seems to become detached. More 

 than five individuals are consequently never found on one stolon. 



B. Further Development of the Buds. 



A fact already pointed out in connection with the development of 

 the four primary Ascidiozooids (p. 407) must again be emphasised 

 here, viz., that the longitudinal axis of the stolon does not coincide 

 with the later longitudinal axis of the developing buds, but rather 

 appears to lie at right angles to it (Figs. 258, 267). The longitudinal 

 axis of the segments of the stolon changes into the dorso-ventral axis 

 of the developed bud. The axis which becomes the later longitudinal 

 axis of the body is one which we may imagine as cutting the 

 longitudinal axis of the stolon at right angles, i.e., from the neural 

 side to that of the genital strand. The ends of such an axis would 

 be indicated by the rudiments of the branchial and a trial apertures. 



From the ectoderm of the stolon is developed the integument of the 

 bud which, on its outer surface, secretes the cellulose mantle. Accord- 

 ing to SEELIGER, this secretion takes place here in the way described 

 for other Tunicates by SEMPER, HERTWIG and others. When, there- 

 fore, SALENSKY emphasises the fact that the first rudiment of the 

 cellulose mantle is yielded by the Cyathozooid, the four primary 

 Ascidio/ooids at first taking no part in the secretion of the cellulose 

 substance, his remark applies only to the first stages of development. 

 The further enlargement of the cellulose envelope proceeds from the 

 Ascidiozooids. 



The branchial and atrial apertures (Figs. 253, 259, *) first appear as 

 simple ectodermal invaginations. These invaginations fuse with the 

 wall of the branchial sac or atrial cavity, the apertures breaking 

 through later at the points of fusion. Near the point at which the 

 branchial aperture forms, the branchial sac (i.e., the entoderm) gives 

 off the first bud-like rudiments of the crown of tentacles that en- 

 circles the entrance to this part of the alimentary canal (Fig. 253, t). 



The entoderm-sac of the bud first forms the branchial sac or 

 pharynx. It has already been mentioned (p. 488) that this is cruci- 

 form in transverse section (Fig. 257), two of the outgrowths being 

 directed upward and two downward. The upward outgrowths are 

 -separated from each other by a median fold in which can be recognised 



