SALPIDAE— FORMS WITH COVERING FOLDS. 



I i:i 



begins to assume a simpler form (Fig. 210 B, ») ; at still ater stages 

 it is found as a cell-mass running obliquely downwards and forwards; 

 within tins mass a cavity appears which communicates anteriorly with 

 the respirator; cavity ( Fig. 222). The part lying nearest to the aper- 

 ,,„,. of the cen tral canal represents the rudiment of the ciliated pH 

 ( f) while the blind end that is directed backward and upward forms 

 the ganglion proper (»). In the course of development these two- 

 sections of the neural rudiment become more sharply marked oft from 

 one another by a constriction. At the same time, in the ganglionic 

 part of the rudiment, the central canal becomes segmented, breaking 

 up into three consecutive cerebral vesicles, a condition first noticed 

 by Kowalevsky, and similar to that 

 seen in S. democratica (p. 429). In 

 later stages the cerebral rudiment 

 becomes completely separated from 

 the ciliated pit, and the two rudi- 

 ments shift apart, although they seem 

 still to remain connected by a nerve- 

 strand that runs forward from the 

 brain to the ciliated pit. The central 

 cavity of the cerebral rudiment disap- 

 pears, and the interior of the rudiment 

 then seems rilled with punctate nervous 

 tissue (Leydig's " Punktsubstanz "). 

 A process running towards the surface 

 leads to the development of the eyes, 

 in which single cells of the ganglion 

 become changed into elements sensi- 

 tive to light, while other cells of the 

 most superficial layer become tilled 

 with pigment (p. 430). No details 

 are known of the development of the 

 paired auditory vesicles which lie in 

 contact with the brain, and were first 



observed by H. Mullek and further described by Todako (No. 

 107). The ciliated pit, by the folding of its walls, assumes a com- 

 plicated form approaching that of the same organ in the adult (Fig. 



214,5,/). . 



The pericardial rudiment (Figs. 2 IS, 219 pc), which is originally 

 a cell-strand running from above downward, divides into two parallel 

 strands (Figs. 223, 224 A) ; the anterior strand, near the enteric 



Fig. 223. — Horizontal section 

 through an embryo of Salpa 

 pinnata (after Salensky). b, 

 blastomeres ; <L enteric rudi- 

 ment : ///. mesenchyme-cells ; n, 

 rudiment of the nervous system ; 

 nh, neural cavity; pc, pericardial 

 rudiment ; x, sub - pericardial 

 cell-strand ; z, cells in the lumeu 

 ofthe pharynx (atrium, Brooks). 



