ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 305 



Germinal Localisation in Patella and Dentalium.* — E. B. "Wilson 

 gives an account of experiments on the development of isolated blast* >- 

 meres which establish definitely the principle of mosaic development in 

 the case of these molluscs. The evidence in Patella that the cleavage 

 cells are definitely specified from the time of their first formation, and 

 that they undergo self -differentiation without essential modification 

 through their relation to the other cells, is demonstrated in the cells of 

 the first quartet, at least as far as the l(!-celled stage. The entire first 

 quartet of Patella, when isolated, produces a mass of ectoblast cells, 

 which, though it closes, does not gastrulate, but undergoes essentially the 

 same differentiation as if it formed the upper hemisphere of a complete 

 larva. The foregoing and numerous analogous facts constitute a strong 

 body of prima facie evidence that the entire cleavage-pattern in the 

 molluscan egg represents (with certain specified reservations) a mosaic- 

 work of self-differentiating cells, exactly in the sense of Roux's general 

 conception. Similar conclusions have been arrived at from a study of 

 the germ regions in the egg of Dentalium. 



Cephalopyge trematoides.f — Elise Hanel describes under this title 

 a new Nudibranch which Chun found on a voyage to the Canaries and 

 recorded % as a species of Phyllirho'd. Some specimens were found free, 

 and two were found attached by a ventral suctorial process under the 

 head to colonies of Halistemma. Hand's more detailed observation of 

 this interesting form shows that it cannot be regarded as a species of 

 Plii/llirhoe. It has a foot-gland, for instance, and the anus opens on 

 the head ; it is a connecting link between the divergent Phyllirhoidaj 

 and the other Nudibranchs. 



5. Lamellibranchiata. 



Artificial Parthenogenesis in Mactra.§ — K. Kostanecki finds that 

 the addition of potassium chloride to the sea-water induces some degree 

 of parthenogenetic development in the ova of Mactra, and he has 

 studied the details of nuclear change in the formation of the directive 

 and segmentation divisions. He confirms Boveri's conclusion that the 

 artificial stimulus evokes the formation of new centres in the cytoplasm, 

 corresponding physiologically to the centrosome normally introduced by 

 the spermatozoon. In certain conditions the artificially induced directive 

 divisions are exactly like those in fertilised ova. The formation of the 

 division-centres for the segmentation spindle occurs apart from the 

 centriole left after the liberation of the second polar body ; it seems to 

 arise de novo in intimate association with the nuclear framework. The 

 formation of this " intra-nuclear spindle," without polar radiation, with- 

 out central granules, with the two poles defined simply by the conver- 

 gence of the spindle-fibres, and leading on by "intra-nuclear karyokinesis " 

 to the appearance of two daughter-nuclei, must be regarded as one of 

 the most striking phenomena in the artificially induced parthenogenetic 

 process. 



* Journ. Esper. Zoology, Baltimore, i. (1904) pp. 1-72, 107-2G8. 



t Zool. Jahrb., xxi. ' 1905) pp. 451-66 (2 pis.)- 



X S.B. Akad. wiss. Berlin. 1888, p. 28. 



§ Bull. Iutemat. Acad. *ci. Cracovie, 1904, pp. 70-91. 



Jane 21st, 1005 Y 



