REPRODUCTION BY GERM-CELLS 



277 



number, and how both are obviously regulated in relation to the 

 difficulties which stand in the way of each sperm-cell before it can 

 reach the ovum. In some species the sperm-cells are very large, in 

 others extremely small. In the genera Daphnia, Lynceus, and others, 

 copulation occurs as shown in Fig. 6^; the sperm-cells (sp) are 

 liberated by the male into the capacious brood-cavity of the female, 

 which at the moment is closed to some extent by the abdomen of the 

 male, in reality closed only partially at the posterior end and at 

 the sides. It seems inevitable that a large proportion of the male 

 elements should stream out again and be lost because of the violent 

 movements of both animals. Accordingly, we find that the sperm- 

 cells are only about the 

 hundredth part of a 

 millimetre in length 

 and of round or rod- 

 like form, and are 

 voided in multitudes 

 into the brood-cavity. 

 Fig- 66, /, [J, and h, 

 show such cells in dif- 

 ferent species, as they 

 occur in the testes to 

 the number of many 

 thousands. But in all 

 the species in which the 

 brood-cavity is closed, 

 and in which therefore 

 there is not such a 



Fig. 66. Spermatozoa of various Daplinids. a, Sida. 

 h, Byihotrephes. c, Baphnella. d, Bloina p'^radoxa. e, 

 Moina rectirostris. f, Eurycercus lamellatus. g, Alonella 

 2njgm(ea. h, Peracantha truncata. All magnified 300 



serious loss of sperm- times, 

 cells, the elements are 



much larger, and at the same time less numerous. They are largest 

 and least numerous in species of genera like Daphnella, PolypJiemus, 

 and Byihotrephes, in which the males have a copulatory organ, so that 

 the possibility of loss of the male cells is excluded. Thus the round, 

 delicate, and viscid sperm-cells of Byihotrephes (Fig. 66, b) are more 

 than a tenth of a millimetre in length, but they are developed in pro- 

 portionately smaller numbers, so that more than twenty are never 

 found in the testis, and often only six or eight, while in copulation 

 only from three to five are ejected. But as there are only two eggs 

 to be fertilized at a time, and as the male cells are expressed into the 

 brood-cavity directly upon the eggs, so that they immediately adhere 

 to them, this small number is amply sufficient. 



