INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 431 



Parthenogenesis in the Ostracoda.* — Dr. Wcismann considers 

 parthenogenesis to; be essentially a unisexual and not an asexual 

 process of reproduction. It is derived from the regular sexual method, 

 being dependent for its occurrence on certain conditions which render 

 a rapid multiplication desirable. Hitherto it has not been observed 

 in the Ostracoda, although they are exposed to the same conditions — 

 among others the periodical drying up of the medium in which they live 

 — which render a rapid renewal of their colonies necessary in the case 

 of the Daphnida). Hence as parthenogenesis prevails in the latter group 

 it was to be expected that a similar provision would be made for the 

 Ostracoda also, especially as, like the Daphnioids, they are endowed 

 with all the organs necessary for such a process. 



Dr. Weismann's experiments made to test the truth of this assump- 

 tion have met with remarkable success. It is found that one species, 

 Cypris incongniens, could be reared for nine months, reproducing 

 solely by parthenogenetic means ; thus, female individuals which were 

 isolated at their birth, produced only similar females which were 

 similarly treated, and so on. In colonies also the multiplication went 

 on without the appearance of a single male. The female possesses a 

 large reccptaculum seminis, hence it is probable that at certain times 

 or places fertilization does take place ; but sperm has never yet been 

 found in this organ. In bisexual colonies of Candona Candida, Ci/pris 

 punctata, ovum, and Cyprois monacha every female (even half grown) 

 had its reccptaculum full of spermatozoa, and this when there was 

 no great preponderance of males present ; it therefore appears to 

 be the rule that when males are present at all, they impregnate all 

 the females ; and so the discovery in any colony of an adult female 

 with an empty rccoptaculum would seem of itself to argue that par- 

 thenogenesis prevails in that colony. 



Besides Cypris incongniens, unisexual colonies of C. fuscata, C. 

 vidua, and C. rcptans also occur, and in tliese instances also neither a 

 male nor any impregnated females have been found, although all 

 species except the last, which was not fully developed, had perfect 

 receptacula. Candona Candida has been found to present colonies of 

 two sexes in spring and summer, and in winter only females with 

 empty pouches and ripe ova, which appears to indicate an alternation 

 of methods as possibly a rule in the Cyprididfo. At present appearances 

 rather tend to the conclusion that certain colonies maintain one method 

 of reproduction regardless of season ; for C. fuscata has solely female 

 colonies at a time when other species are reproducing bisexually ; 

 while on the other hand Cyprois monacha appears never to reproduce 

 by parthenogenesis. Thus a fundamental diflference exists between 

 this family and the Daphnoid.no in the absence of a regular alternation 

 of reproductive methods in the one, and its constant occurrence in the 

 other family. 



As to the origin of the parthenogenesis, although it is only an 

 hypothesis that it is a derived procoss in the Daphnioids, yet tho 

 Cypridida) seem to prove by their fully developed receptacula and 

 other accessory organs that they have received it merely as a modifi- 

 cation of the older bisexual method. 



♦ ' Zool. Anzciger,' iii. (1880) p. 82. 



