PHENOMENA ACCOMPANYING FERTILIZATION 317 



eggs from animals of widely different phyla which usually undergo 

 fertilization before development. 



The chief biological interest of parthenogenesis centers in the 

 nuclear phenomena. Under ordinary conditions of fertilization 

 two polar bodies are formed by the maturing egg and with their 

 formation the number of chromosomes is reduced to one-half so 

 that egg pronucleus and polar body nuclei are haploid. It follows, 

 therefore, that in artificial parthenogenesis all tissue cells of the 

 body are haploid. The same phenomenon occurs, naturally, in the 

 development of the drone honey bee, or of the male rotifer and may 

 be referred to hereafter as Type 1. In the great majority of par- 

 thenogenetic eggs, however, the second polar body is not formed 

 and the nucleus remains diploid as for example in parthenogenetic 

 aphids or female rotifers; this may be designated Type 2. A third 

 possibility, in theory, would be cases where two polar bodies are 

 formed which, with the pronucleus, are haploid but the egg becomes 

 diploid by later fusion of the pronucleus with one of the polar 

 body nuclei. This which may be called Type 3 has not been estab- 

 lished with certainty in any metazoon but was suggested as a possi- 

 bility by Boveri (1887) and described by Brauer (1893) as one type 

 of parthenogenesis in the eggs of Artemia. 



In Protozoa many cases of so-called parthenogenesis have been 

 described some of which fall in line with one or another of the three 

 types in Metazoa as outlined above. These phenomena may be 

 grouped under two headings— so-called endomixis of Woodruff and 

 Erdmann (1914) and autogamy, a widely used term in connection 

 with Protozoa. 



A. Endomixis.— Under this term Woodruff and Erdmann (1914) 

 described "a complete periodic nuclear reorganization without cell 

 fusion in a pedigreed race of Paramecium.'" At regular intervals 

 of approximately thirty days they found that the old macronucleus 

 of Paramecium aurelia gives rise to buds or fragments which are 

 absorbed in the cytoplasm. There appears to be some difference 

 in the details of macronucleus fragmentation between individuals 

 in 1914 and more recent individuals. Thus Woodruff and Spencer 

 (1922) find that ribbon or skein formation prior to fragmentation 

 and characteristic of conjugation, which was very rare in 1914, 

 had become much more common in 1921. Each of the two micro- 

 nuclei divides twice, forming S products some of which form new 

 micronuclei, some new macronuclei. The possible combinations of 

 nuclei and their relations are shown in Fig. 161. Later, Erdmann 

 and Woodruff (1916) demonstrated a similar periodic reorganiza- 

 tion at intervals of approximately sixty days in Paramecium cauda- 

 tum. In this case the single micronucleus divides three times, 

 forming 8 nuclei, 4 of which become macronuclei, 2 possibly degen- 

 erate and 2 persist as new micronuclei. 



