130 FUNDAMENTALS OF CYTOLOGY 



includes. The inference that cleavage accompanies internal differentia- 

 tion but does not produce it is borne out by the striking fact that the 

 larva of an annelid worm has been seen to develop its characteristic form 

 and structure to a certain stage even when the cleavage divisions were 

 suppressed altogether by treatment with KCl. Further discussion of 

 these phenomena would lead us into the fascinating but extremely difficult 

 field of developmental mechanics. 



Parthenogenesis. — The development of an egg into an embryo 

 without syngamy is called 'parthenogenesis. An individual so derived is a 

 parthenote. This mode of reproduction occurs frequently in lower 

 animals, notably insects, lower crustaceans, and rotifers. In some 

 species, parthenogenesis is the only mode of reproduction, male indi- 

 viduals being unknown, while in others a series of parthenogenetic 

 generations is succeeded under the proper environmental conditions by 

 individuals that reproduce sexually. Eggs of several other groups, 

 including echinoderms, mollusks, amphibians, and even mammals 

 (rabbit), in which parthenogenesis does not normally occur, have been 

 made to undergo parthenogenetic development, at least to a certain 

 stage, by treating them variously with hypertonic sea water, fatty acids, 

 alkaloids, foreign blood serum, and a number of other agencies. Only 

 exceptionally do these artificially induced parthenotes reach the stage of 

 metamorphosis or of sexual maturity. 



Some animal parthenotes have the gametic chromosome number, 

 normal meiosis having occurred in the development of the egg. In bees 

 and ants, for example, such eggs are capable of either sexual or partheno- 

 genetic development: when syngamy occurs the result is a diploid female, 

 whereas parthenogenesis leads to the development of a monoploid male. 

 In the majority of cases the parthenotes are diploid, as in aphids and 

 rotifers. The eggs developing in this manner arise from oocytes in 

 which there are no meiotic divisions, or there may be a single division 

 which is equational in character. These diploid eggs are usually incap- 

 able of fertilization. 



There is every reason to believe that the parthenogenetic mode of 

 development in these organisms has been derived from the normal sexual 

 cycle, involving a suppression of meiosis in the diploid type and an adjust- 

 ment of development to a different nuclear constitution in the monoploid 

 type. The suppression of meiosis is suggested by the various observed 

 conditions intermediate between meiosis and ameiosis and between 

 facultative and obligatory parthenogenesis, and also by the fact that in 

 certain plants parthenogenesis occurs only after a failure of meiosis. 

 That the ability to undergo somatic development with a single genome 

 may have been slowly acquired is indicated by the fact that monoploid 

 parthenotes occasionally encountered among plant and animal species 



