412 



TUNICATA. 



up in the formation of the pericardial vesicle, but is continued pos- 

 teriorly. This mesodermal strand and the corresponding strand of 

 the left side (the so-called left pericardial strand, Fig. 203, pc') do 

 not for the present develop further, but at a later stage, when the 

 Ascidiozooid has become independent and prepares to give rise to 

 fresh buds, these strands pass over into the proliferating stolon which is 

 developing and, on either side of the endostyle-process (entoderm-tube) 

 that is also continued into the stolon, form the mesoderm-rudiment 

 of the latter structure (Salensky). According to Salensky, there- 

 fore, the paired pericardial strands of the Ascidiozooid give rise, on 

 the right, to the pericardial vesicle of the Ascidiozooid, and, in their 

 further course, to the two mesoderm-strands of the proliferating stolon 

 (p. 486). 



By the development of the peribranchial sacs and the above-men- 

 tioned mesoderm-formation in the lateral parts of the embryo, the 

 primary body-cavity is divided into two longitudinal sinuses, one 

 above and the other below the alimentary canal (Fig. 203), called by 

 Salensky the supra-intestinal and the sub-intestinal blood- sinuses. 

 In the region of the sub-intestinal sinus the mesenchyme- cells collect 

 (Fig. 203 A, g) and unite to form the genital strand belonging to the 

 posterior region of the Ascidiozooid. In later stages, according to 

 Salensky, a lumen appears in this strand (Fig. 203 C, g), round 

 which the cells become arranged 

 like an epithelium, but the lumen 

 disappears again in the course of 

 further development. The genital 

 strand not only represents the 

 genital rudiment of the Ascidio- 

 zooid in which it appears, but gives 

 rise to the genital strand of the 

 proliferating stolon of this zooid, 

 as will be described later (p. -±84). 

 The rise of the genital organs in 

 the first four Ascidiozooids has 

 recently been described in detail 

 by Seeligek (No. 76a). As it 

 resembles that of the corresponding 

 organs in the zooids that develop 

 later we must refer the reader to 



the description given on p. 493, merely adding here that, in the 

 first four Ascidiozooids, the ovary degenerates. It was well-known to 



/ 



Fig. 204. — Transverse section through 

 an Ascidiozooid of Pyrosoma (after 

 Salensky). <ib, diapharyngeal band; 

 ed, intestine ; i, rudiment of the 

 branchial aperture (ectodermal 

 thickening) ; oe, oesophagus ; p, peri- 

 branchial cavities. 



