372 SUMMARY OF CURRENT RESEARCHES RELATING TO 



morphological value of ova, and they give to the gonad which produces 

 them the aijpearance of a hermaphrodite organ. It may be that, in the 

 testes of the higher Prosobranchs, they represent the ova prodiiced by 

 the gonad of the hermaphrodite types which were separated from the 

 Prosobranch stock at the same time as that at which the higher or 

 Monotocardate Prosobranchs made their appearance. 



Fertilization in Helix aspersa and Arion empiricorum.*— Dr. P. 

 Garnault has been led by his researches to recognize two distinct actions 

 in the process of fertilization in these two molluscs. One consists in the 

 impulsion given to segmentation which i?, indeed, produced by the 

 spermatozoon, but which may be caused by an external and mechanical 

 excitation, as has been shown by the experiments of Tichomirolf on the 

 ovum of Bomhyx mori. The other consists in the assurance of the 

 transmission of characters ; this may, in organisms with a diffused 

 nucleus, merely consist of the fusion of two bodies. In higher organisms, 

 where there is a vesicular nucleus, the fecundating individual, the 

 spermatozoon, fuses with the yolk by its protoplasmic part, but its nucleus 

 (head) is the point of departure of a nuclear formation (male pronucleus) 

 which becomes relatively enormous by borrowing its materials from 

 the ovum. 



In fact, if in the parthenogenetic ovum a single nucleus is developed, 

 evidently at the expense of the egg, two are developed in the fecundated 

 egg, and they are equivalent in mass to the single nucleus ; they are 

 both developed at the expense of the egg, but one of them has arisen 

 from the head of the spermatozoon. They elaborate in common sub- 

 stances destined to fuse later with the protoplasm; from the point of 

 division they behave as in the parthenogenetic egg, but the nuclear 

 substance has received the influence of the male. It is impossible to 

 say what is the precise moment of fertilization ; all the phenomena 

 which occur are of the nature of cellular actions, the morphological 

 manifestations of which are simple in the lower organisms which have no 

 nuclei, and are more complicated in those which possess one. In the 

 latter case one can only admit that the nuclear phenomena which are 

 produced at the moment of fertilization, constitute the essential part of 

 the phenomenon, while the nucleus is the sole substratum of the 

 essential characters of the individual. 



Neurology of ProsobrancMata.t — Dr- J. Brock has made an 

 investigation of the nervous system of a number of Prosobranch 

 Molluscs. He comes to the conclusion that the great majority of 

 Prosobranchiata are, so far as the development of the terminal plexus of 

 the anterior margin of the foot is concerned, higher than the Ehipido- 

 glossa. There appears to be a definite connection between the delimita- 

 tion of an anterior portion of the foot as propodium and the better 

 development of the ganglionic plexus which is so characteristic of the 

 anterior margin of the foot. This is the more remarkable as the 

 whole formation of the propodium in the few families that are pro- 

 vided with it is so different that its appearance must, in many cases 

 at least, have been independent of the rest. Although the author 

 does not propose to rehabilitate Huxley's division of the Gastropod 

 foot into pro- , meso-, and metapodium, which has, indeed, been sliown by 



* Zool. Anz. vj; , xi. (1888) pp. 731-6 ; xii. (188D) pp. 10-15 and .^3-S. 

 t Zeitschr. f. Wiss. Zool., xlviii. (1889) pp. 67-S3 (2 pis.). 



