736 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



given. Not even Brock's own figures lend support to this conclusion. On 

 the contrary, Semper believes that Eanzaud was right in deriving the 

 spermatheca and its stalk from a splitting of the primary genital duct. 

 Nor do Brock's figures seem to Semper to support his statement that the 

 spermatheca arises at the same time as, or slightly before, the male duct 

 begins to separate off. To Semper it appears that the male genital duct of 

 Brock does indeed separate o& from the primary genital duct, not, however, 

 to disappear, but to become spermatheca and stalk. And further, that 

 what Brock calls the spermatheca is no such thing, but a glandular 

 appendage of the atrium and neck of the penis, homologous with the 

 mucous gland and Cupid's dart-sac. 



According to Brock the spermatheca is a development from the penis. 

 But in a dissection of Helix pomatia made many years ago in Prof. Semper's 

 laboratory the following interesting relation was observed and afterwards 

 confirmed. At the point where the oviduct and vas deferens begin to 

 separate a short duct arises, which connects the origin of the separate vas 

 deferens with the stalk of the spermatheca. The latter in this case has 

 retained its primitive connection with the hermafphrodite duct. The very 

 variable diverticula on the stalk of the spermatheca, seen in so many 

 Stylommatophora, are thus explicable as nothing more than the upper ends 

 of the " male genital duct " of Brock. These have separated from their 

 connection with the common genital duct, but have not degenerated to the 

 extent observed in a normal Helix pomatia or Limax, where they have dis- 

 appeared except at that point where the spermatheca arises. Each of the 

 aforesaid diverticula is a rudiment of the upper (proximal) end of the 

 separated male duct. Semper's opinion, then, is that the spermatheca is a 

 lateral diverticulum from the male duct as this becomes gradually sepa- 

 rated from the hermaphrodite duct, and that the variable diverticula on the 

 stalk of the spermatheca are so many retrogressive developments of the 

 upper end of the male duct, and that the connection between the herma- 

 phrodite duct and the spermatheca above described is a frequent persistent 

 structure (Hemmungsbildung). The paper concludes with a hit at 

 naturalists who are ready to explain all divergent facts in terms of their 

 theory without analysis of the facts involved. 



Central Nervous System of Acephalous MoUusca.* — Dr. B. Eawitz 

 was led to study the acephalous Mollusca from the supposition that the 

 simplicity of the arrangement of their central nervous system would be 

 very suitable for investigating the significance of the so-called commissures, 

 and the origin of the nerves. He examined Pholas dactylus, Mya arenaria, 

 Cardium edule, Unio pictorum, Mytilus edulis, and others. 



With regard to his histological results, the following are the most 

 important points ; the central nervous system contains no apolar cells, 

 unipolar cells are the most and bipolar cells the least common; multipolar 

 cells are less frequent than the former, and more frequent than the latter. 

 Some of the unipolar cells have their process going directly to the peri- 

 phery, but the peripheral processes of all other cells sink into and become 

 lost in the medullary substance ; the multipolar cells are collecting-cells, 

 and their medullary process is the homologue of Deiter's process. 



The medullary substance is formed (a) of a central nervous plexus, 

 which is formed by the intermingling of the products of the division of the 

 medullary processes ; (6) of nerve-fibrils which are formed from the meshes 

 of the nerve-plexus ; and (c) of a substance which resembles the nervous 

 medulla of the Vertebrates, which forms the characteristic myelin, and 



* JenaJsche Zeitschr. f. Naturwiss., xx. (1887) pp. 384-460 (5 pis.).' 



