No. I.] GERM-LAYERS IN CLEPS/NE. WJ 



Stage is reached, consisting at first of a few small cells lodged 

 between the micromeres and the macromeres, and derived in all 

 probability from the latter. Bergh is inclined to the belief that 

 this mode of origin holds true of the Rhynchobdellidae as well as 

 the Gnathobdellidae. He finds in the egg of Clepsine certain 

 cells beneath the blastodisc, in a position which corresponds 

 perfectly to that of the first entoderm cells in Nephelis, and 

 hence infers that they represent the entoderm of Clepsine, 

 although he has not traced their development into this layer. 



Notwithstanding the remarkable difference in the position of 

 the " residual " yolk, which distinguishes the two types of leeches 

 represented by Clepsine and Nephelis, it must be taken for 

 granted, so long as no positive proof to the contrary has been 

 produced, that the mesenteron arises in essentially the same 

 manner in both cases. I have ascertained enough of the his- 

 tory t)f the cells in Clepsine, which Bergh identifies with the 

 entoderm of Nephelis, to convince me that he is partly right on 

 this point; but it is an error to suppose that they constitute the 

 whole, or even the larger part, of the entoderm in Clepsine, and 

 the observations of Kowalevsky and Biitschli fall very far short 

 of establishing such a conclusion in the case of Nephelis. 

 Kowalevsky gives no figures, and offers only the following brief 

 sketch of the germ-layers, which he illustrates by referring to 

 Rathke's plates : — 



" Was die Scheidung der Blatter anbetrifft, so ist dieselbe 

 schon auf der Fig. 13, Taf. I., bei Rathke zu entdecken ; die 

 oberen Zellen, jf, bilden das obere oder sensorielle Blatt, die 

 zwischen denselben und den grossen Furchungskugeln lieg- 

 enden — das Darmdrilsenblatt (Figs. 14 and 15), und die drei 

 grossen Kugeln — das mittlere Blatt. Weiter umwachsen die 

 Zellen des oberen Blattes die ganze Masse von alien Seiten ; 

 die Darmdriisenblattzellen wachsen sehr schnell, verlieren ihr 

 kdrniges Aussehen, werden zu grossen hellen Zellen und 

 drangen dabei die grossen Zellen des mittleren Blattes nach 

 hinten, Welche durch Abschniirung zwei Zellenreihen bilden, 

 die bekannten Keimstreifen der Nephelis sind." (No. 8, p. 3.) 



Kowalevsky's interpretation of the three large segments — 

 the homologues of the entoblasts in Clepsine — as mesoblasts, 

 has been corrected by Biitschli, and his statement on the origin 

 of the entoderm is as indefinite as it is brief. 



