936 SPECIAL PHYSIOLOGY. 



Trematode Entozoa, in many Gasteropods, and in a few branchiated 

 Annelida, the male and female products are developed in one individual, 

 which is then said to be monoecious or hermaphrodite. In the Medusae, 

 and in the Entozoa, the germ-cells are fertilized by the sperm-cells of 

 the same animal ; but in the Snails, and other Pulmo-gasteropods, 

 there is an interchange of office, one individual fertilizing the ova of 

 another, and having its own ova fertilized in return. 



In the remaining animals reproduced by true ova, viz., the higher An- 

 nuloida and the Annulosa, the Molluscoida, Mollusca, and Vertebra ta, 

 the reproductive elements are found in separate individuals belonging to 

 opposite sexes. Such animals are named dioecious. The ovum is then, 

 in some cases, as in Fishes and Amphibia, fertilized without, but in 

 other cases, as in Reptiles, Birds, and Mammalia, within, the body of 

 the female or ovigerous parent. In this ovum, when fertilized, the 

 embryo is developed, undergoing a series of important changes, which 

 constitute the process of evolution or transformation. 



Some curious examples of the coexistence in the same individual, but 

 at different seasons, of a sexual with an apparently nonsexual mode of 

 reproduction, have been met with, in the lower Classes of animals. 

 This presents us with the various forms of parthenogenesis, or develop- 

 ment by so-called unfertilised ova. This mode of reproduction is illus- 

 trated in the Aphis, amongst Insects, in the female of which, one act 

 of fertilization is sufficient for a long succession of distinct reproduc- 

 tive acts. Another most striking example is exhibited by the Bee, as 

 was first observed by Dzierson, and afterwards by Siebold, Berlebach, 

 Leuckart, Owen, and others. The ova of the Queen-bee are deposited 

 by her, in the cells of the comb, and in that act, according to the size 

 and form of the cell, she either fertilizes the ovum, or not. This is 

 accomplished by her permitting, or preventing, the escape of a small 

 quantity of fluid from a sac in the interior of her, body, named the 

 spermotheca, which has been previously charged, by the act of the male 

 bee, with fertilizing fluid, during flight in the air. If the ovum be 

 fertilized, it produces a working-bee, i. e., an undeveloped female, any 

 one of which, by abundance of feeding, may become a queen-bee. But 

 if the ovum be not fertilized by the fluid of the spermotheca, it pro- 

 duces only a drone, or male. This latter result may be brought about 

 experimentally, either by interruption of the communication between 

 the spermotheca and the oviduct, or by the effects of a temperature 

 low enough to destroy the properties of the fertilizing fluid. So also, 

 if the wings of the queen-bee be cut, she remains with the sac un- 

 charged with the fertilizing fluid, and her eggs, which she will then de- 

 posit all the same, produce only drones. Moreover, a working-bee, 

 not fed up to the condition of a queen-bee, may deposit eggs, which, 

 not having been fertilized in the ordinary way, produce only drones. 

 In the Bee, therefore, the phenomenon occurs, of an ovum undergoing 

 development, without obvious direct fertilization. Hence the name 

 parthenogenesis. Similar phenomena have now been observed in many 

 other Insects. 



In certain remarkable cases, a sexual generation by true fertilized 

 ova, or germ-cells^ may occur, together with reproduction, by appar- 



