502 The Determination of Sex 



Parthenogenesis 



In Chapter 4 we pointed out that upon some occasions eggs 

 may develop into new individuals without ever having been fer- 

 tilized. We stated there that this is a regular occurrence in such 

 organisms as bees and wasps, but that it may be induced in 

 many other animals and even in some vertebrates by some sudden 

 change in environmental conditions. It is beyond our scope to 

 describe examples of induced parthenogenesis, but the reader is 

 referred to Morgan's Experimental Embryology and to other 

 works in the fields of ontogeny and embryology. 



Parthenogenesis may be either haploid or diploid. In haploid 

 parthenogenesis meiosis occurs normally, and haploid eggs are 

 produced which then proceed to develop without being fertilized. 

 In diploid parthenogenesis the meiotic divisions are abnormal in 

 some features, and the eggs which are formed and later develop 

 without the assistance of any male cell have two genomes. Hap- 

 loid parthenogenesis is a constant feature in the life cycle of 

 the group known as the Hymenoptera, which includes the bees 

 and wasps, and always results in male offspring. Diploid par- 

 thenogenesis is a regular feature of the life cycle of some animals. 

 In some of the lower animals reproduction is almost solely by 

 this method, with the result that males occur very rarely if at 

 all; in other animals two or more generations of females are 

 produced by this method, after which sexual forms are produced. 



PLANTS 



Sexual reproduction in the higher plants is complicated by the 

 presence in the life cycle of two separate and distinct generations, 

 as we mentioned in Chapter 4. Male and female gametes are 

 present and arise respectively from male and female gametophyte 

 plants. These in turn develop from the male and female spores, 

 better known as the microspores and megaspores, and these 

 spores form on the sporophyte plant on micro- and mega- 

 sporophylls, which, in the angiosperms, are found in an or- 

 gan known as the flower. The microsporophylls are the stamens 

 and the megasporophylls are the carpels, and one or more carpels 

 always forms an enclosed pistil. In most species of the flowering 

 plants both stamens and pistils are present in the same flower. 

 Such flowers are perfect, bisexual, or hermaphrodite. Flowers, 



