438 



TUNICATA. 



to be replaced by a secondary follicle-cavity that appears in the same 

 place.* 



The body-cavity (SALENSKY'S secondary follicle-cavity) separates 

 the rudiments of the organs laterally and ventrally from the body- 

 wall. In the latter we can now distinguish an external layer, the 

 ectoderm (Fig. 218 A, ec), from an inner layer, the cells of which 

 wander into the body-cavity, filling it with a niesenchyrne. We are 

 consequently enabled to recognise in this inner layer a part of the 

 mesoderm-r udiment . 



If, in order to obtain a correct idea of the relative positions of the 



organ-rudiments, we 

 examine horizontal 

 sections (Fig. 219), we 

 find in them a remark - 

 able cruciform figure, 

 the enteric rudiment 

 (d) forming the trans- 

 verse portion of the 

 cross, while the neural 

 rudiment (n) and the 

 pericardial rudiment 

 (pc) form the longi- 

 tudinal portion. It 

 is characteristic of 

 the Salps now under 

 consideration that the 

 enteric rudiment (d) 

 originally appears to 

 develop chiefly in a 

 transverse direction . 

 At the same time, it 

 is evident that the 

 first parts of the 

 enteric rudiment 

 (pharynx) to develop 

 are its future lateral 

 portions so that there 



* [BROOKS considers that this second cavity is probably the original cavity 

 of the follicle opened a second time by the growth of the surrounding parts. 

 He would thus derive the body-cavity in Salpa from the primary follicular 

 cavity, the latter he believes to represent the cleavage-cavity of the Ascidiacea. 



FIG. 219. Horizontal sections through two embryos of 

 Salpa pinnata, made in the direction of the line a-a 

 in Fig. 218 A (after SALENSKY). b, the blastomeres ; 

 d, enteric rudiment ; d', layer covering the intestine ; 

 ec, ectoderm ; n, middle part, n', lateral parts of the 

 nerve-rudiment ; p, lateral parts of the placenta ; pc, 

 pericardial rudiment; pi, blood-cavities of the placenta. 



