426 



TUNICATA. 



itself from the ectoderm, giving rise to a cleft which gradually widens 

 to form the rudiment of the primary body-cavity (I}. A cavity rises 

 in a similar way within the entodermal cell-mass, this being the first 

 rudiment of the alimentary canal, especially of the branchial sac 

 (pharnyx, Fig. 213, d). This chamber, when it first appears, con- 

 sists of two lateral cavities connected across the middle line by a 

 narrower part (Fig. 213 , k).* The dorsal ingrowth that partly 

 separates the two halves of the enteric cavity must be regarded as 

 the rudiment of the gill (Fig. 213 B, k). It becomes separated from 



m j'. 



FIG. 213. Transverse sections through two ontogeuetic stages of Salpa democmtica- 

 mucronata (schematic, original), a, outer lamella of brood-sac ; d, rudiment of the 

 alimentary canal ; ec, ectoderm ; k, rudiment of the gill ; I, primary body-cavity ; 

 ms, mesoderm ; p, rudiment of the placenta (remains of the follicle) ; t, the epithelial 

 covering (basal plate) of the placenta ; x, tissue of the placenta ; z, large marginal 

 cells of the placental tissue. 



the enteric rudiment, by the fusion of the at rial cavity, which 

 develops between it and the ectoderm, with the projecting lateral 

 diverticula of the pharyngeal cavity. The atrial cavity of Salpa is 

 derived by TODARO (No. 113) from an ectodermal invagination. The 

 gill, which is originally a solid ingrowth of cells, changes later into 



* [Investigations both on this and other species show that, in every case, 

 the atrial cavity is the first to appear. HBIDEE and KOBOTNEFF agree in 

 describing it as a single cavity from its earliest origin, while BROOKS ascribed 

 a paired origin similar to that found by SALENSKY for the atrium and, on this 

 account, BROOKS concludes that SALENSKY mistook the atrium for the pharynx. 

 The pharynx appears below this as either a single or paired cleft in the 

 embryonic mass, and the gill arises from the horizontal septum between 

 these two cavities (atrium, and pharynx), by the appearance in it of a pair of 

 laterally placed longitudinal slits. ED.] 





