276 - Multicellular Animals, Especially Man 



rived from the paternal parent, and half 

 from the maternal parent. 



Shortly after the zygote nucleus is formed, 

 the egg begins a long series of divisions, 

 giving rise to the man) cells of the embryo 

 and adult of the next generation (Fig. 15-2). 

 All these divisions are mitotic, and conse- 

 quently every cell of the embryo and adult 

 receives a diploid set of chromosomes equally 

 derived from the two parents. 



Parthenogenesis. Having contacted an 

 egg, the sperm performs two other functions: 

 (1) it contributes its chromosomes to the 

 zygote nucleus, and (2) it activates the de- 

 velopment of the egg. These two functions 

 are separable, to some extent, as is demon- 

 strated by the fact that if a sperm is removed 

 just after it has made contact with an egg, 

 fertilization proper cannot occur. Neverthe- 

 less such an egg may develop, at least par- 

 tially, even in the absence of the paternal 

 chromosomes. The contact of the sperm ini- 

 tiates a visible reaction that sweeps over 

 the surface of the egg. This activation reac- 

 tion involves the discharge of a bioelectric 

 current, which precedes the lilting of the 

 fertilization membrane. As a result of activa- 

 tion, the egg becomes nonreceptive to other 

 sperm; and the metabolism of the egg is 

 changed and accelerated. In due time the 

 activated unfertilized egg begins to divide; 

 in some cases it may continue development, 

 forming an embryo, or even an adult — al- 

 though such individuals are not entirely 

 normal in appearance and vitality. 



Activation represents the normal response 

 of a ripe egg to stimulation. In fact, a variety 

 of stimuli are now known that can substi- 

 tute for the contacting sperm. Some eggs can 

 be activated by mechanical pricking, or by 

 drastic temperature changes, or by chemicals, 

 or by sudden changes of the osmotic condi- 

 tions in the surrounding medium. Appar- 

 ently the unfertilized egg is all "set" and 

 ready to develop. Normally, the stimulus for 

 starting development is provided by the 

 sperm that first succeeds in contacting the 

 egg surface; but a variety of other strong 



stimuli may inadvertently "jolt the mechan- 

 ism" into activity. 



The development of an egg, in the ab- 

 sence of fertilization, is called parthenogene- 

 sis. Parthenogenesis may be artificial — that is, 

 induced experimentally; or it may be natural, 

 in that it occurs normally in the reproduc- 

 tion of some species. An organism produced 

 by artificial parthenogenesis is haploid, at 

 least in the sense that it lacks paternal 

 chromosomes. Moreover, the development 

 of such organisms tends to be abortive. A 

 few animals (for example, frogs) have been 

 reared into adults by careful nursing; but 

 such adults are small and delicate, and they 

 are never able to reproduce. 



Natural parthenogenesis is regularly en- 

 countered in certain worms, insects, and 

 crustaceans; they may be either haploid or 

 diploic! in nature. Haploid parthenogenesis 

 is common to the ants, bees, and wasps (p. 

 (i59). The unfertilized eggs of these animals 

 always develop into males, which, being 

 haploid, produce sperm, not in the usual 

 fashion, but by mitosis. The fertilized eggs 

 of these species always develop into females, 

 and these diploid females finally produce 

 haploid eggs by meiosis, as in other animals. 



Diploid parthenogenesis is exemplified by 

 the aphids, or plant lice. Commonly the fe- 

 male aphid produces diploid reproductive 

 cells by mitosis. These diploid cells are called 

 parthenogonidia, rather than eggs. During 

 the spring and summer months, the parthe- 

 nogonidia develop into other parthenoge- 

 netic females; and in this way a number of 

 asexual generations may be produced. In the 

 fall, however, the parthenogonidia develop 

 into males and sexual females; and these 

 forms of the organism produce haploid 

 gametes, by meiosis, in the usual fashion. 

 Further development cannot occur until fer- 

 tilization takes place. If aphids are raised 

 in a greenhouse, under uniform conditions 

 throughout the year, the sexual generation 

 may never appear; and in certain species, 

 males are unknown even under natural con- 

 ditions. Thus parthenogenesis, although 



