DOLIOLIDAE ASEXUAL REPRODUCTION. 



471 



shaped projection rising from an ectodermal depression ; in cross- 

 section, this is found to be composed of seven parallel cell-strands 

 (Fig. 242) four of which (/* and y>) are 

 arranged symmetrically in two pairs, 

 while the other three (n, z, and //o are 

 unpaired. GROBBEN and ULJANIN differ 

 considerably as to the origin and signifi- 

 cance of these seven strands which, with 

 the ectoderm that envelops them, form 

 the primary rudiment of the buds. 

 These two authors agree that two cones 

 grow downwards from both the pharyn- 

 geal and the atrial cavities of the 

 "nurse" (Fig. 241, p/i and d), enclosing 

 between them a mesoderm-mass (/>*), the 

 origin of which has already been traced 

 (p. 387;. see also Fig. 182 B, <l and 

 HIS', p. 387). According to ULJANIN, 

 the stolon is thus originally composed 

 of five strands, two pharyngeal and two 

 atrial strands and a middle mesoderm- 

 strand (Fig. 241). The number of these 



strands becomes augmented to seven when the atrial strand 

 becomes bent on itself, its reflexed end 

 giving rise to a new pair of strands (cl"). 

 The fusion of this pair, according to 

 ULJANIN, yields the future neural rudi- 

 ment (n), while the pharyngeal strand (/>) 

 yields, by dehiscence, an unpaired middle 

 strand (x) in which this author sees the 

 rudiment of the pharyngeal cavity of the 

 bud. The unpaired strand (m) is said to 

 represent the pericardial rudiment, while 

 the pharyngeal strands (/;) change into 

 the genital rudiment, and the atrial 

 strands (k) into those of the muscle-plates. 

 According to GROBBEN, on the con- 

 trary, the pharyngeal strands (j>) represent 

 the rudiment of the pharyngeal cavity 

 and intestine of the bud, the atrial strands 

 (/) the later rudiment of the atrium, and the unpaired mass ~x 



Fi<;. 24.1. Young stage of de- 

 velopment of the proliferating 

 stolon (rosette -like organ) of 

 holiohm/ Mnlleri (after ULJA- 

 XTN). cl, atrial wall of the 

 parent-animal ; d', ont-growth 

 of the atrial wall ; d", bent 

 portion of the same ; m, aggre- 

 gation of mesoderm forming 

 part of the rosette-like organ ; 

 />, pericardial vesicle ; ph, 

 pharynx of the parent ; ph', 

 outgrowth of the wall of the 

 pharynx. 



FKJ. 242. Transverse section 

 through the ventral stolon or 

 a primitive bud of Ihtliolum 

 (diagram after GROBBEN and 

 IIJANIN). ec, ectoderm ; k, 

 muscle-rudiments ; m, peri- 

 rardial rudiment ; n, neural 

 rudiment ; p, genital rudi- 

 ment ; ./.-, pharyngeal rudi- 

 ment . (Ui,J.\xix). 



