ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 271 



two species ; the first two cleavages result in an anterior, a posterior, 

 and two lateral blastomeres, and the subsequent cleavages are closely 

 similar to what Conklin has described in Crepidula. 



While such facts as the three generations of ectomeres and the 

 origin of the primitive mesoderm cells point to hereditary predetermina- 

 tion, the regularity of certain planes of cleavage and their angles point 

 to extrinsic physical factors, probably in great part capillary. 



Monograph on Pleurobranchidse.* — Prof. A. Vays^iere completes 

 his monographic account of this family with a discussion of the genera 

 Oscaniopsis and Pleurobranchsea, and their species. 



Ganglion-cells in the Gullet Musculature of Pulmonates.f — Dr. 

 H. Smidt points out that hitherto ganglion-cells in the muscular tissue 

 of Invertebrates have been found only in the heart. He has examined 

 the buccal cavity and gullet of Helix, and finds scattered ganglion-cells 

 in the musculature of the radula and cartilaginous cushion, arranged 

 along the fibrils which arise from the buccal ganglion. They are ovoid 

 in form, are surrounded by a nucleated investment, and are mono-, bi-, 

 or multi-polar ; they appear to be quite analogous to the ganglion-cells 

 previously described in the heart of various Invertebrates. 



5. Lamellibranchiata. 



Development of Dreissensia polymorpha Pall .J — Dr. J. Meisen- 

 heimer, who adopts this form of the name in preference to the more 

 familiar Dreissena, has followed this development in great detail. The 

 process of segmentation strongly recalls that of Unio ; but while Dreis- 

 sensia, which, despite the present habitat, is essentially a marine form, 

 retains the trochoph ore-stage, Unio has lost this, and its development 

 has become correspondingly modified. The author, on the basis of his 

 investigations, discusses the value of the " germ-layers " in Mollusca in 

 general, and the phyletic significance of the trochophore. From his 

 point of view the development consists in a progressive differentiation 

 of organs, which appear in rudiment even at the first segmentation- 

 stage. From the primitive indifferent cell-masses the organs are dif- 

 ferentiated by invagination and by proliferation, especially by the latter. 

 As differentiation goes on, two influences are clearly discernible, the 

 inheritance of ancestral peculiarities, and the recently acquired special 

 peculiarities, and it is the interaction of these two which gives the de- 

 velopment its peculiarities. What is called " entoderm " is merely the 

 rudiment of the mid-gut, and this is homologous from Coelentera to 

 Mollusca. Generally, the germ-layers are primitive rudiments (Primitiv- 

 anlagen) which may be simple, as when they give rise to one organ 

 only, or compound, when they give rise to several. In regard to the 

 phyletic position of the trochophore, the author believes that this larval 

 form undoubtedly links molluscs closely to annelids, and that both arose 

 from a trochophore-like ancestor. The early trochophores resemble one 

 another closely ; but while in the annelid the posterior region becomes 

 elongated and segmented, in the mollusc the larval body increases 

 equally in all directions except in the velar region. That the point of 



* Ann. Sci. Nat. (Zool.), xii. (1900) pp. 1-85 (6 pis.). 



t Arch. Mikr. Anat, lvii. (1901) pp. 621-31 (1 pi.). 



t Zeitschr. wiss. Zool., lxix. (1901) pp. 1-137 (13 pis. and 18 figs.). 



