72 SUMMARY OF OUBBENT RESEARCHES RELATING TO 



that lie has seen also a similar specimen of L. foetidus ; he points out 

 that it is not a case of budding, and, remarking that the clitellum 

 only became apparent just before the hinder ends were lost, says that, 

 if the two facts are correlated, it only shows that asexual reproduction 

 (to which reproduction of parts is pro tanto comparable), is not com- 

 patible or contemporaneous with sexual reproduction. It is possible 

 that the phenomenon of a bifid tail is not rare among earthworms, but 

 only one such case has before been put on record. 



Development of BrancMobdella.* — Prof. W. Salensky commences 

 with a note on the species of BrancMobdella ; of these he has used 

 that which is parasitic on Astacus leptodactylus, the eggs of which are 

 of a size which lends itself to the preparation of sections ; these eggs 

 are attached to the gills of the crayfish -by a very delicate pedicle, and 

 are invested by two membranes, the outer of which forms a thick 

 chitinous capsule, is very hard and very elastic, and cannot be 

 detached from the living egg. It is necessary to make use of chromic 

 acid, which causes the membrane to swell, and after some time, to 

 soften. The inner or vitelline membrane is so delicate, and so closely 

 applied to the yolk that it cannot be removed without injuring the 

 egg itself. 



The yolk consists, as in various Annelids, of a large number of 

 highly refractive granules; the nucleus is spherical and small, and 

 contains several nucleoli ; the first changes that occur in a freshly 

 deposited egg affect the form and situation of the nucleus ; the author 

 was unable to observe the copulation of the two pronuclei and 

 the formation of the first segmental nucleus. Segmentation, while 

 recalling in some particulars that of Nephelis, differs in some essential 

 points ; the blastomeres are from the first formation of two macro- 

 meres asymmetrical, and this asymmetry becomes more and more 

 marked ; there is, further, great individual variability in the form and 

 distribution of the blastomeres, and this even in the earliest stages of 

 segmentation. The micromeres divide much more rapidly than the 

 macromeres, and we meet therefore with epiboly. The variations 

 cause, as may be supposed, considerable difficulty in the orientation 

 of the eggs. 



Eegarding the process of segmentation as a whole we find essential 

 differences between BrancMobdella and Glepsine or NepJielis ; the 

 position of the poles and of the ovular axes as compared with the poles 

 and axes of the embryo is different ; there are differences also in the 

 history of the macromeres, for, in BrancMobdella, they multiply 

 during the whole period, and they give rise to the endodermal cells, 

 to those which appear to correspond to the neuroblast of Glepsine, 

 and to the ectodermal cells. The author regards the differences as 

 due to differences in biological conditions, and suggests that such, and 

 the absence or presence of the gastrula-stage ought to be explained as 

 being cenogenetic. 



The external modifications of the embryo are next considered ; 

 when segmentation is ended, the embryo becomes pyriform, and the 



* Arch, de Biol., vi. (1885) pp. 1-61 (5 pis.). 



