224 THE CONTINUITY OF THE RACE 



species, including the hagfish and the oyster, the sperm cells and ova of a 

 particular individual are not matured at the same time. 



Unisexual reproduction (parthenogenesis). In several orders of 

 insects and in certain other invertebrate groups there is a special and 

 rather rare type of sexual reproduction called parthenogenesis (Greek, 

 -parthenos, "virgin," and genesis, "origin"). A part or all of the eggs are 

 able to develop without fertilization. Such eggs are clearly germ cells; 

 they have developed and matured in an ovary, have undergone meiosis, 

 and are often outwardly and even microscopically indistinguishable from 

 ordinary eggs. Unlike ordinary eggs, they undergo embryonic develop- 

 ment without any stimulus from or union with a sperm. 



There is much evidence that parthenogenesis is a derived condition, 

 developed in groups that were once bisexual but that have become able to 

 dispense with fertilization, wholly or in part. Males may be temporarily 

 (or in a few cases permanently) absent. In some of the aphids or plant 

 lice, for example, males appear only in the fall, to fertilize bisexual eggs 

 that survive the winter in a resting state; during the spring and summer 

 there are several generations consisting only of females. Here we see an 

 alternation between bisexual and unisexual generations. In other in- 

 stances, as in the honey bee, the eggs will develop either with or without 

 fertilization. At her single union with the male the female receives a 

 store of spermatozoa, which she can release or withhold at will as the eggs 

 pass through the oviduct. The fertilized eggs produce females (workers 

 or queens), the unfertilized eggs males (drones). 



Artificial parthenogenesis. Among a few animals — starfishes, sea 

 urchins, and frogs, for example — in which the eggs will not normally 

 develop without fertilization, it has been found that unfertilized eggs 

 can be made to develop by certain special treatments. Such treatments 

 include placing the eggs in various solutions for a certain length of time 

 or, in the case of the frog egg, pricking the egg membrane with a needle 

 that has been dipped in frog serum. Usually the individuals produced 

 by such methods are feeble and often fail to complete their normal 

 development, 1 but these experiments indicate that one of the roles per- 

 formed by the spermatozoon is to stimulate development. Another role, 

 that of contributing paternal, inherited qualities to the offspring, will be 

 considered under gepetics and evolution. 



BREEDING HABITS IN THE METAZOA 



Bisexual reproduction is by all odds the most common and important 

 way of producing new individuals. It is the sole method available to the 

 vertebrates and most of the higher and intermediate metazoans, and it 

 is at least occasionally utilized by nearly all the lower metazoans that 



1 Adult frogs, however, have been obtained by artificial parthenogenesis. 



