ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 773 



there is no dorsal proboscis. The Aductifera have the wall of their 

 mid- and hind-gut formed by a syncytium. The contractile vesicle is 

 not a special appendage of the cloaca, and the nervous system has no 

 lateral palps. In the Ductifera the enteric- wall-cells are polygonal 

 and have a membrane, the vesicle is a special appendage of the cloaca, 

 and there are (except in Conoclrilus) two dorsal and two lateral 

 palps. Male Philodinaids have never yet been found. 



Two distinct species of Gregarines are to be found in the intestine 

 and body-cavity of Gammarus pulex. They are both Polycystids, and 

 have three distinct body-regions. 



Mollusca. 



Fertilization in Arion.* — Continuing his researches on the re- 

 productive relations of Gasteropods, Herr. G. Platner has studied the 

 phenomena of fertilization as exhibited in Arion empiricorum. After 

 noting some general points, such as the variable period between ferti- 

 lization and egg-laying, and the appearance of the freshly laid ovum, 

 Herr Platner notes how two yellow cross folds in the uterus of Arion, 

 just below the albumen gland, mark the position where the ovum, at 

 the stage with two segmentation spheres, becomes surrounded by a 

 firm homogeneous layer of albumen. At suitable epochs the whole 

 upper portion of the uterus was removed, hardened for half an 

 hour in chromic-osmic-acetic acid, washed, imbedded in c'elloidin, cut 

 in sections, and stained lightly with hematoxylin and then with 

 safranin. 



The cavity of the uterus included many spermatozoa, which 

 almost always lay in the plane of the section, at right angles to the 

 long axis, an arrangement obviously advantageous both for snail and 

 investigator. In the stages examined the polar cells were already 

 formed, and most frequently separated. Their appearance, when 

 attached and when isolated, is described. Three were always present, 

 evidently the result of the indirect division of one of the original 

 two. They possessed distinct membranes in which spermatozoa had 

 occasionally penetrated. A full history of the observations as to 

 polar cells in Gasteropoda is given. Herr Platner regards the process 

 as a necessary preliminary to, but independent of fertilization, con- 

 sisting essentially on the removal of nuclear constituents to be replaced 

 by the male nucleus. 



After describing the egg before and after fertilization — and there 

 is no marked difference — he notes the penetration of the sperm, usually 

 at the vegetative pole, and therefore furthest from the germinal 

 vesicle. The head becomes surrounded by a rapidly widening radiate 

 figure, but the rays are less developed on the side turned towards 

 the nucleus of the ovum. The head itself is slightly modified, is 

 without sharply defined nucleus, and is surrounded by a clear space. 

 It still remains in direct contact with the tail, which has to a large 

 extent been drawn into the yolk, where it is readily noted by its very 

 marked staining. Only one spermatozoon entered at once, if a second 



* Arch. f. Mikr. Anat., xxvii. (1S8G) pp. 32-72 (2 pis.). 



