The Organization of " Daphnia Pulex " 137 



Tlie most remarkable features in the reproduction of Daphnia are 

 that they present an example of parthenogenesis or " virgin genera- 

 tion," (the female being capable of reproduction without the inter- 

 vention of the male) ; and that they lay two kinds of eggs. Male 

 Daphnias are extremely rare, not more than two having been met 

 with during these investigations, although some 600 Daphnias have 

 been passed under the microscope. This paucity is explicable in 

 view of the well-ascertained fact, that a single sexual intercourse 

 suffices to fertilize the female, not only for the remainder of her own 

 life, as in Cyclops, but also all her female progeny for several genera- 

 tions, indeed so long as the surroundings remain constant. A few 

 observations will best illustrate these points. 



A female having ova in the dorsal cavity was isolated on the 4th 

 of February. The young born on the 9th were each placed in a 

 separate glass tube. She gave birth to a second brood on the 1 7th, 

 and a third on the 25th. The same day ova were placed in the 

 cavity, but on the 28th a depression or bulge appeared in the dorsal 

 portion of the carapace (a certain precursor of death), and she lingered 

 on until the 12th of March, the development of the embryos mean- 

 while being arrested. One of her young, born on the 9th of Feb., 

 hatched a brood on the 6th and another on the 26th of March, &c. 

 Another incubating female isolated on the ] 0th of January, hatched 

 on the 17th, again on the 5th of February, and died from confervce on 

 the 8th. One of her young born and isolated February 5th, therefore 

 agamic, bore young on the 27th of March, on the 4th, 11th, 16th, 

 22nd, and 29th of April, &c. Numberless examples might be 

 adduced, but these will suffice, if it be understood that in all cases 

 each individual was completely separated at birth from its species, so 

 that sexual intercourse was impossible. 



Besides these agamic eggs so abundantly developed, the Daphnia, 

 occasionally produces protected, ephippial " eggs. The most 

 posterior ovum in each ovary always differs from the rest, in having 

 its nucleus surrounded by a series of brown corpuscles, and eventually 

 it becomes an ephippial egg ; but, according to the investigations of 

 Lubbock (2), the presence of the male is necessary to mature it, 

 otherwise it gradually fades away : when it continues, the remaining 

 ova slowly disappear. 



( To he concluded in our next.) 



(2) Phil. Trans, of Koyal Society, 1857, pt. Ill, 



