No. I.] GERM-LAYERS IN CLEPSINE. 131 



corresponds in position to the archiamphiaster (" amphiaster de 

 rebut"), and which is traversed by the first cleavage-groove. 

 This pole is correctly identified with what I have called the 

 " oral pole " in Clepsine. In view of these facts it is very re- 

 markable that Salensky should regard the first cleavage-groove 

 as equatorial. I fail to see a single fact which could be urged 

 in support of such a view. The relation of this groove to the 

 " clear spot" is conclusive evidence that it is meridian, precisely 

 as it is in the Hirudinea and other annelids. If further proof be 

 required, it may be found in the development of the eight-cell 

 stage, which corresponds in every prominent feature with the 

 same stage in Clepsine, Nephelis, and Rhynchlemis. This is a 

 point of primary importance ; for if the first cleavage were equa- 

 torial, the subsequent cleavages would be just as little comparable 

 with those of Clepsine as the first, and the axial orientation 

 would be radically different in the two cases. Precisely how 

 the embryonic axis is related to the first two cleavage-planes is 

 not clear; but, judging from Salensky's figures, it appears to 

 be the same as that of the types before considered. There is a 

 large macromere, from which the first micromere arises, and 

 which divides, asymmetrically, prior to the division of the other 

 three macromeres. It is this macromere that I would identify 

 with the " posterior macromere " in Clepsine. According to 

 Salensky (pp. 20-21) the two segments into which this ma- 

 cromere divides, participate equally in the formation of all the 

 germ-layers. That is, they are not destined to play unlike parts, 

 as they do in Clepsine, the one, representing a primary meso- 

 blast, the other a neuro-nephroblast, but each alike gives rise 

 to entodermic, mesodermic, and probably ectodermic elements. 

 " Elles donnent naissance aux cellules niesoentodermiqiies et ne 

 representent point des ebauches speciales ni du mesoderme, ni 

 du systeme nerveux, comme c'est le cas chez Clepsine." 



There appears to be the same confusion here as we found in 

 Nephelis, in regard to the origin of the entoderm and meso- 

 derm. The earlier " meso-entodermic " cells in Branchiobdella 

 appear to correspond to the first deep cells which arise beneath 

 the micromeres in Clepsine and Nephelis, and which, as I have 

 shown, are purely entodermic. Salensky has found cells 

 which are unmistakably the homologues of the teloblasts of 

 Clepsine ; but he has failed to trace their origin and to deter- 



