412 SUMMARY OF CURRENT RESEARCHES RELATING TO 



accessory nuclear body is no longer visible, having probably remained 

 as a portion of the nucleus at the last division. In the formation of 

 the spindle, the origin of the fibres from the unstained substance 

 was clearly seen, and this is, in developing ova, contained in the 

 germinal spot. In progressive changes the spot exhibits a variable 

 but small number of clear vacuole-like bodies ; a clearer stained and 

 a darker portion become distinguishable, the latter representing 

 Van Beneden's " corpuscule germinatif." Meanwhile, the surround- 

 ing protoplasm is becoming modified* within the meshes of the 

 network of fine granular threads, yolk-granules appear, from the 

 centre outwards. This differentiation of the protoplasm is for the 

 most part effected at the cost of the nutritive cells. The mature ova 

 no longer form a continuous peripheral layer, but lie in scattered 

 clumps, with the spermatogonia between them. The intermediate 

 substance between the clumps is composed of the connective-sub- 

 stance cells described by Leydig or the plasma-cells of Brock. 

 With the growth of the alveoli these gradually degenerate and 

 disappear. The nuclei of the alveolar wall which furnish the reserve 

 germs, do not, therefore, originate from the intermediate substance, 

 but from the sexual cells. The basal cells are formed from sexual 

 cells with granular nuclei, which after serving for a time as the 

 nutritive centres of spermatocyte groups, degenerate and disappear. 



3. Oogenesis in Helix. — In Hdix the ova are formed at intervals, 

 there is no strict peripheral disposition, the primitive ova multiply 

 with mitosis, and their final form exhibits a single germinal spot. 

 The function of the nutritive cells is beautifully seen. At first 

 directly apposed to the ova, they come gradually to lie in cavities, 

 and are finally indistinguishable from the surrounding protoplasm. 

 The assimilation takes place rapidly. These so-called yolk-nuclei 

 are, in this case, at least, in no way parallel to the accessory nuclear 

 body of sperm-cells. Herr Plainer agrees with other investigators 

 as to the absence of a vitelline membrane. 



Parasitic Gastropods.* — Drs. C. F. and P. B. Sarasin state that 

 the Linckia multiformis of Ceylon has two parasites, one internal and 

 one external, both of which are prosobranchiate gastropods. The 

 ectoparasitic form is always found on the lower surface of an arm, 

 and is so set that the anterior portion of the right margin of the 

 shell has free communication with the outer world. The surface of 

 attachment is almost as large as the wide mouth of the shell, and it 

 is fixed by a number of elevations which make their way into the 

 cutis of the Linclda ; this surface is not, however, the true foot, for 

 its centre is occupied by the pharynx, which acts like a proboscis and 

 makes its way perpendicularly into the skin of the host. The true 

 foot is a small semilunar fold on the hinder surface, and the remnant 

 of the velum forms a second semilunar fold on the anterior surface. 

 There are no tentacles, but auditory vesicles are present. The 

 muscular pharynx has a pumping action, and the radula is wanting. 

 There are well-developed salivary glands, which, possibly, have an 



* Zool, Anzeig., ix. (1885) pp. 19-21. 



