THE VITAL PROPERTIES OF THE CELL 295 



multiply parthenogenetically. When nutriment is withheld from 

 the Phylloxera vastatrix, the winged sexual forms, as Keller (VII. 

 26) has shown experimentally, soon make their appearance, and 

 fertilised eggs are laid. 



In many cases, especially amongst the lower organisms, the 

 need for fertilisation is only relative. 



When the female gamete of the Alga Ectocarpus (VII. 51) comes 

 to rest, for a few minutes it becomes receptive. " If the egg is 

 not fertilised at this time, it draws in its flagella completely, be- 

 comes spherical, and excretes a cellulose membrane. After from 

 twenty-four to forty-eight hours parthenogenetic germination first 

 begins to make its appearance." Even the male gametes are 

 capable of spontaneous development, although in a less degree 

 than the female. After they have swum round for several hours, 

 they finally, as Berthold states, come to rest, " but only a portion 

 of them develop slowly into very weak and tender embryonic 

 plants, whilst the remainder become immediately, or after the 

 course of one or two days, disintegrated." 



A very peculiar facultative relation is seen in Bees, whose eggs, 

 whether fertilised or not, develop into adults. But the unfertilised 

 eggs produce drones, and the fertilised, female Bees (working 

 and queen-Bees). Sometimes, as is stated by Leuckart, herma- 

 phrodites are derived from eggs which were fertilised too late for 

 the development in the male direction to be entirely set aside. 

 The possibility of accelerating, or, on the contrary, of delaying the 

 need of fertilisation in sexual cells by interference from with- 

 out, throws light upon the phenomena of parthenogenesis and 

 apogamy, which we are now about to discuss in detail. 



a. Parthenogenesis. In most cases sexual cells, both in the 

 animal and vegetable kingdoms, perish quickly, unless they are 

 fertilised at the right time. Although they consist of a substance 

 which is eminently capable of development, yet if this one con- 

 dition fails they cannot develop. 



Till a short time ago the majority of scientists were so con- 

 vinced of the impossibility of the spontaneous development of 

 the egg-cell, that they received the theory of parthenogenesis 

 with incredulity, because they perceived in it an offence against a 

 law of nature. And, indeed, it may be accepted as a law of 

 nature for mammals, and for the majority of other organisms, 

 that their male and female sexual cells are absolutely incapable 

 of development by themselves. Any single species of mammal 



