ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 455 



absorbed. The sperm-head, breaking away, moves with its base in 

 advance towards the egg-aster. The centrosome, at first invisible, 

 becomes distinct and often large. Sometimes the sperm-centrosome of 

 Limnsea divides into two while on its way toward the egg-nucleus, a pro- 

 cess prophetic of the existence of the centrosomes at either pole of the 

 first cleavage-spindle. 



Beasonably satisfactory proof that the first cleavage-spindle is wholly 

 of spermatic origin is found in the fact that the incipient cleavage-spindle 

 involves the sperm-nucleus first, and that the egg-nucleus is never in- 

 volved in the spindle before the sperm-nucleus is entirely drawn upon it. 



Abbreviated Development in Nudibranchs.* — Prof. A. Vayssiere 

 notes that Prof. Paul Pelseneer has found that in Genia cocksi the larval 

 stage is suppressed, and the young form leaves the egg as a miniature 

 adult. It is of interest to note that Vayssiere observed the same pheno- 

 menon in Pelta coronata (Buncina hancocki) in 1887. The adults laid eggs 

 in captivity, and the nidamental ribbon was found to contain relatively 

 few (20-30) eggs, which were of considerable size (1-1 '4 mm. in dia- 

 meter), and contained a large amount of yolk. Segmentation is partial, 

 and there is no trace in the subsequent development of veliger or nautili- 

 form shell, though at one period the embryo is covered with cilia and 

 rotates in its membranes. On emergence the young form displays the 

 characters of the adult, and the intestine still contains a large quantity 

 of nutritive yolk. Details of the development are promised shortly, but 

 it is noted that the velum is probably represented by a tuft of cilia 

 above the blastopore. 



Distribution of Opisthobranchs.t — Prof. A. Vayssiere notes that 

 the Opisthobranchs on the oceanic coasts of France (Atlantic and the 

 Channel) are for the most part northern forms, while those on the Medi- 

 terranean coasts are predominantly southern. Some lists of genera are 

 given to illustrate this. In the Bay of Biscay the northern and southern 

 forms occur side by side. 



Arthropoda. 



a. Insecta. 



Basal Segments of Insect's Leg.J — Mr. -k. B. Walton discusses the 

 series of segments and sclerites which enter into the composition of the 

 base of the leg. These are the (1) trochanter, (2) articulating with it the 

 coxa genuina, (3) the meron trochantin or posterior lateral part articu- 

 lating with the epimeron, and (4) the antecoxal piece. His conclusions 

 are the following : — In Hexapoda and Chilopoda the coxa is composed 

 of two more or less fused segments, coxa genuina and meron ; the ante- 

 coxal piece results from the chitinisation of the membrane connecting 

 the coxa with the sternum. The trochantin probably originated from a 

 lateral portion of the same membrane. Audouin erroneously homolo- 

 gised the lateral margin of the posterior coxa in Dytiscus circumflexus 

 with the trochantin of the prothorax and mesothorax ; the trochanter 

 represents a distinct segment of the legs. The meron and coxa genuina, 

 together with their corresponding basal segments, epimeron and epi- 



* Zool. Anzeig., xxiii. (1900) pp. 286-8. 



t Comptes Eendus, cxxx. (1900) pp. 926-7. 



X Amer. Nat., xxxiv. (1900) pp. 267-74 (6 figs.). 



