EMBRYOLOGY OF CLICPSINE. 271 



sucker. Robin ('^^) ascribes a very insignificant role to these 

 cells. They are called " ectoderme dorsal,'"' in consequence of 

 their having an analogous origin with the cells, which really 

 form the dorsal ectoderm in Nephelisj although here (Clepsine), 

 only the two small median cells {or^) are said to enter into the 

 ectoderm, " Nous verrons en effet que ces cellules, situees 

 comme celles de I'ectoderme dorsal des Nephelis, ne donnent 

 qu^ut) nombre restreint de cellules a Tarriere de I'ectoderme sur 

 les ClepsineSj par subdivision nlterieure de deux fl?' eutre elles 

 (^*), pendant que les six autres restent longtemps sans changes 

 (!), a la place qu^ occupera I'anus." They disappear by atrophy 

 some time after the embryo hatches. Leuckart (-,Vi!4) has 

 expressed the opinion that these cells represent primordial 

 kidneys^ comparable with those described by Gegenbaur for 

 Gastropods. He now concurs, however, in the opinion that 

 the segmental organs have no connection whatever with these 

 cells.^ 



Kowalevsky (14) found two cells at the hind end of the germ- 

 bands (" Keimstreifen ") in Euaxes. (See his figs. 1], 12, K, 

 PL III.) He regarded them as mesoblasts. In the case of 

 Tubifex {1%), five such cells were found, and the germ-bands 

 were composed of /?;?e longitmiinal lines of cells . As the cleavage 

 and neurulation (Rauber) in Euaxes and Tubifex are closely 

 similar to the same in Clepsine, it seems very probable that 

 these cells are of the same character as the neuroblasts before 

 described. The five longitudinal lines of cells observed in 

 Tubifex would then be nerve-cells, derived from five neuroblasts, 

 just as four such lines are derived from four neuroblasts in 

 Clepsine. 



(7t) Formation of Entoplasts. — About the time the germ- 

 bands begin to form, a number of free nuclei appear in 

 the surface of the entodermal blastomeres {a, b, c.) These 

 nuclei (fig. 37) are very distinct in the egg of C. complanata, 

 and it is remarkable that they have so long escaped obser- 

 vation. They appear like dark spots in the opaque yolk, just 

 as the unclei of the neuroblasts or of the blastodisc. They 

 are oval, oblong, or biscuit-shaped, and measure '02 to '05 

 mm. At the time of appearance th^y number three to four 

 in each blastomere, two or three of which occupy the position 

 seen in the figure, while the others are near the lower pole. 

 They are encircled by white rings, such as are generally seen 

 around the nuclei of the neuroblasts. The substance of these 



' Ratzel and Warschawsky {-^^-^^X^x^ described two cells at the liind 

 ends of the germ-bands (Luinbiicus agricola). The exact relation of the 

 same to the bauds was not ascertained. 



