ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 649 



medicinalis there is a dorsal sinus, which Burner regards as coelomic 

 and comparable to the dorsal cavity in Clepsine. 



I "Clepsine has a pair of blood-vessels, dorsal and veutral, which are 

 both inclosed in coelomic spaces, corresponding to similar vessels in 

 Chtetopods, but apparently not represented in the other leeches named 

 above. As in Chaitopods these vessels arise ia Clepsine in connection 

 with the process of coalescence which unites the primitive segmental 

 cavities above and below the gut. Peculiar to Clepsine are the " cardio- 

 blasts" — two rows of mesoderm cells arising in connection with the 

 primitive segments, and shunted from left and from right towards the 

 middle line both ventrally and dorsal ly. 



The cells which in other leeches form the '• botryoidal tissue" be- 

 come in Clepsine " excretophores" fat-cells, and Stapelzellen. The ne- 

 phridia arise as in other leeches, as Bergh has described, in a manner 

 comparable to that observed in Chsetopods. Burger shows further that 

 the origin of the reproductive system in Clepsine is similar to that in 

 the Gnathobdellidae. 



Brain of Phascolosoma.* — Marcel A. Herubel gives an account of 

 the minute structure of this brain which lies between the two dorsal 

 retractor muscles, euveloped in a fibrous — muscular and connective — . 

 sheath. Three regions are distinguishable and are minutely described. 



The most interesting result is, that the brain in question seems to 

 consist of a syncytium of nuclei witljin an anastomosing network, the 

 punctated substance of which seems to be the immediate centripetal 

 element. It is, therefore, in the network and not in the " cell," that 

 we must look for the seat of the essential nervous function. In short, 

 Herubel's results are against attaching importance to the individuality 

 and fixity of nerve-cells. Nervous function implies correlation. 



Development of Sagitta.f — L. Doncaster has confirmed Hertwig's 

 account, except that in S. bipunctata the head-cavities are formed as 

 Biitschli described (1873). Hertwig probably studied a species with 

 minute head-cavities. 



Sections of the embryo show that in its early stages the nuclei lie 

 at the free ends of the cells, but as development proceeds those of the 

 ventral ectoderm sink into their bases, and in the ventro-lateral areas 

 a great proliferation of nuclei takes place giving rise to the lateral 

 nuclear bands of the ventral ganglion. The cavities of the embryo 

 disappear entirely, the endoderm becomes reduced to a thin septum, the 

 mesoderm to two solid strands, in which most of the nuclei become 

 aggregated dorsally and ventrally, and the cell-protoplasm beueath 

 them becomes converted into the longitudinal muscles. 



The larva is as described by Hertwig, but he failed to observe the 

 mode of formation of the posterior transverse septum, which arises be- 

 tween the genital cells of each side as they migrate from the splanchnic 

 mesoderm across the body-cavity to the body-wall. This migration 

 takes place at the time of the reappearance of the ccelom, and the 

 septum is probably formed from the mesodermic envelopes of the 

 genital cells. 



* Comptes Rendus, cxxxiv. (1902) pp. 1603-5. 

 t Proc. Cambridge Phil. Soc, xi. (1902) p. 267. 



