ANNELIDA 325 



the division of the neuronephroblast (Fig. 151 B, C) ; in that case the 

 circumcrescence of the macromeres by the layer of small cells and the 

 germ bands would seem to produce an epibolic gastrula, an interpretation 

 that was in fact given by Balfour. The deep layer of the germ bands 

 arising from the mesoblasts would then be the mesoderm, though its 

 superficial layer also, by becoming overgrown with the small cells, soon 

 comes to lie inside. These processes recall to a certain extent those in 

 Jihynchelinis, in which OligochaBte the mesomeres at first lie in the region 

 of the ectoderm and give off to it products of their division. Perhaps 

 more detailed observations on this point will yield greater evidence of 

 agreement. At present the germ bands of the Hirudinea and the meso- 

 dermal bands of the Oligochata are not to be looked upon as homologous 

 structures, for they are composed of different kinds of elements. How- 

 ever, Kleinenberg argues for a participation of the ectoderm in the 

 formation of the mesodermal bands, and Wilson likewise finds these 

 same bands of cells, which form the germ bands of the Hirudinea, even 

 in the Oligochieta (comp, supra, p. 294). 



If such stages of the embryos of Clepsine and Liiinhricits as are shown 

 in Fig. 153 (p. 324) and Fig. 132 (p. 286) are compared, the conclusion 

 is natural that processes which led to such similar structures must have 

 been at the beginning of like nature, even though they are now changed 

 in their details. 



B. Gnathobdellid^. 

 A detailed investigation of the cleavage of the egg of 

 Nephelis has been given by Bütschlt. Nevertheless, owing 

 to the small size of the egg, we are not as accurately informed 

 about the cleavage and formation of the germ-layers in the 

 GnathobdellidcK as about the corresponding processes in the 

 Rhynchohdellidx. At all events, certain differences between 

 the groups seem to exist. 



In Nephelis there also occurs a cleavage stage of four macromeres and 

 four micromeres, though the latter are said not to arise from all four, 

 but from only three, of the macromeres, whereas the fourth, posterior 

 blastomere remains for a time passive. These three macromeres then 

 again give rise to three small cells, which are arranged, as in Clepsine, 

 under the micromeres first formed, and constitute the first entoderm 

 cells. The fourth of the four macromeres now divides into two large 

 blastomeres, which Whitman interj^rets as corresponding to the neurone- 

 phroblast and mesoblast in Clepsine. According to this view, to which 

 Bekgh also inclines, the superficial layer of the germ bands would be 

 derived from the former, the lower layer, on the contrary, from the 

 latter. The fact that the "neuronephroblast" is said to form two 



