286 C. O. WHITMAN. 



(epithelium) arises from a solid cylindrical mass of cells which lies 

 ill the axis of the embryo^ hetv:een the large blastomeres {a, h, and 

 c). These cells are regarded as a prolongation of the "cord of 

 cells '' from which the walls of the pharyngeal atrium and the 

 pharynx are formed. The lumen of the oesophagus gradually 

 extends backwards through the cellular cylinder, and thus a 

 digestive cavity arises lined with the axially placed cells, external 

 to which are the segments, a, h, and c. The same mode of 

 formation is maintained for Nephelis, but incorrectly according 

 to Biitschli (;30). 



In regard to the role performed by the primary blastomeres 

 {a, b, and c), Robin has arrived at conclusions utterly at variance 

 with what is taught by figs. 93 — 95. Soon after exclusion 

 segmentation sets in, beginning with r, which according to Robin 

 occupies at this time the posterior end of the embryo, and ex- 

 tending to a and h. The result is that these blastomeres are 

 broken up into a large number of cells lying externally to the 

 epithelium. The part they take in the composition of the ali- 

 mentary canal is stated by Robin himself thus : — " Ce n'est pas 

 par atrophic qu'ils disparaissent, mais en se segmentant en grosses 

 cellules qui forment /a coucJie mo^enne de Pintestin, et par- 

 , ticulierement hi couche Iiejudique." On the other hand, I have 

 t found that these blastomeres preserve their individuality during 

 1 the entire period of invagination and neurulation, and that no 

 \ cells, save those before mentioned, are ever found in their 

 ! interior. Furthermore, that the entoderm incloses these large 

 I yolk-spheres, instead of developing by an axial extension of the 

 i "oesophageal cord " through the centre of these. 



VII. Circulatory .Apparatus. 



That we do not to-day possess a complete knowledge of the 

 circulation in the Ilirudinea is not the fault of neglect nor of 

 unskilful hands ; for among those whose patience and ingenuity 

 have been taxed by this problem are such men as Cuvier, 

 Moquin-Tandon, Siebold, Joh. Miiller, Leydig, Wagner, Gratiolet, 

 and Leuckart. Clepsine, Nephelis, and Hirudo inedicinaHs have 

 been the principal objects of study. Fillippi, 0. F. Miiller, 

 Grube, Leydig, Bidder, and others have made Clepsine an object 

 of study in this particular. Filippi (36) found only the two 

 lateral lacunae and supposed that these were in direct com- 

 munication wath the digestive cavity. 0. F. I\[iillor (120) saw 

 all the main channels except the median sinus. 



Grube (59, 60) makes no mention of the lateral lacunae, but 

 says the dorsal vessel gives off to either side as many branches 

 as there are lateral diverticula in the stomach. ]--cydig (108), 

 whose account is the most accurate of any that has yet been 

 given, saw all five longitudinal channels and gave a correct 



