SALPIDAE FORMS WITH COVERING FOLDS. 



439 



is at first a paired rudiment of the pharynx only slightly connected 

 in the median line. According to SALENSKY, the first recognisable 

 rudiments of this system of organs are found as two entirely distinct 

 accumulations of cells derived from the wall of the primary brood-sac 

 (wall of the follicle).* In a similar way, in AS', democratica-mucronata 

 also, the rudiment of the pharynx develops in the form of paired 

 cavities (p. 426). Two layers may be distinguished in the enteric 

 rudiment ; an inner layer consisting of deep cylindrical cells (ento- 

 derm) and an external layer, the so-called covering layer of the 

 intestine (d'), in which a few larger blastomeres (enteric blastomeres, 

 Fig. 219 A, b) can still be recognised and which may perhaps be 

 considered as belonging to the mesoderm. 



In horizontal sections the rudiment of the nervous system has at 

 first a curious trilobate form (Fig. 219 A), but this is less marked in 

 later stages. TODARO (No. 107) has given to the two paired lobes 

 (ri) the name of the dorsal disc, 

 and regards them as belonging to 

 the mesoderm. He considers them 

 to be provisional and homologous 

 to the chorda. The pericardial 

 rudiment (pc) is distinguished by 

 the regular arrangement of the 

 blastomeres found in it, two columns 

 of the latter in which the cells are 

 arranged in pairs running through 

 it. Both the pericardial rudiment 

 and the neural rudiment project 

 slightly at their upper ends above 

 the surface of the embryo (peri- 

 cardial and neural projections, Fig. 

 220, ') ; in later stages a dorsal 

 longitudinal furrow runs between 

 them, but as to the significance of 

 these structures we are still in the 

 dark. 



In these stages the embryo lies like a flat disc on the placenta (Fig. 

 218 A). Only its upper surface appears covered by the cap-like 

 ectoderm-layer (Fig. 221, ec). It is not clear in what way the basal 



* [See footnote, p. 426. The above account of the origin of the pharynx 

 appears to be quite inaccurate. In all probability the cavity described by 

 SALENSKY was the atrial cavity, but even that structure is not derived from 

 the wall of the follicle but from the ectodermal blastomeres. ED.] 



FIG. 220. Horizontal section through 

 an embryo of Salpa pinnata (after 

 SALENSKY). d, the two atrial diverti- 

 cula of the enteric rudiment (gill-slits 

 of BROOKS) ; k, rudiment of gill ; 

 n, neural rudiment ; %', so-called 

 neural projection ; z, cells within the 

 lumen of the intestine (invaginated 

 somatic layer of the follicle, BROOKS). 



