296 THE CELL 



would unquestionably die out, if its male and female individuals 

 did not unite in the act of generation. Nevertheless, it cannot be 

 stated as a general law of nature, that ova are always incapable 

 of development unless they are fertilised. 



Both in the vegetable and the animal kingdoms, numerous in- 

 stances occur of cells being formed in special sexual organs, which 

 were, as far as we can judge by their design, originally destined 

 to develop by means of fertilisation as eggs; but which have sub- 

 sequently lost their need for fertilisation, and in consequence 

 behave exactly like vegetative reproductive cells, that is to say, 

 like spores. 



Only female specimens of Ghara crinita, one of the higher 

 Algae, are to be found in Northern Europe. In spite of this, ova, 

 which develop without fertilisation into normal fruits, are formed 

 in the oogonia. 



Still more instructive are the cases of parthenogenesis which 

 occur in the animal kingdom. They have been observed chiefly 

 in small animals belonging to the Arthropoda, in Rotatoria, Aphides, 

 Daphnidw, Lepidoptera, etc. At one time females produce in 

 their ovaries only ova which develop without fertilisation, and at 

 another the same individuals form those which require fertilisa- 

 tion. Ova, with such different physiological attributes, generally 

 differ in appearance. The parthenogenetic ones are exceptionally 

 small and poor in yolk, and in consequence develop in a shorter 

 time and in greater numbers ; whilst, on the other hand, those 

 which require fertilisation are much larger and contain much 

 more yolk, and consequently require a longer time for their de- 

 velopment. Since the former are only produced in summer and 

 the latter at the commencement of the cold season, they have 

 been distinguished as summer and winter eggs. The latter are also 

 called retarded eggs (Dauereier), because they have to pass 

 through a somewhat lengthy period of rest after fertilisation, 

 whilst the summer eggs (Sulitaneier) immediately enter upon 

 the process of development. 



The development of the parthenogenetic summer eggs, and of 

 the winter eggs, which require fertilisation, may be affected by 

 external conditions. In Aphides, abundant nourishment favours 

 the formation of summer eggs, whilst a diminished supply of 

 nourishment causes the production of ova requiring fertilisation. 

 Daphnidse are also evidently affected by the environment, although 

 the individual factors can be less easily established experimentally. 



