56 PRINCIPLES OF EMBRYOLOGY 



A final point of interest in connection with parthenogenesis concerns 

 the number of chromosomes found in the resulting embryos. The treat- 

 ments which bring about artificial parthenogenesis do not usually cause a 

 complete suppression of the maturation divisions, which would produce 

 diploid eggs, such as those characteristic of the diploid parthenogenesis 

 which is a normal means of reproduction of many species in nature. On 

 the contrary, the activated egg as a rule contains a haploid nucleus or 

 nuclei, and might be expected to develop into a haploid individual. This 

 is indeed what very often happens. There is, however, a well-marked 

 tendency for the diploid chromosome number to be restored. This would 

 happen if the first cleavage division of the nucleus were not accompanied 

 by a division of the cytoplasm, and the two daughter nuclei reunited. 

 The imperfections of the spindle mechanisms developed in artificially 

 parthenogenetic eggs seem often to bring this result about. In fact, in 

 some forms such as frogs, similar irregularities in division often occur at 

 later stages of parthenogenetic embryos, and isolated cells or regions of 

 tissue may arise with many different multiples of the basic chromosome 

 number. 



SUGGESTED READING 

 Tyler 1941, 1948, Rothschild and Swaiin 1949, Runnstrom 1952^. 



