440 INTRODUCTION TO CYTOLOGY 



ined Tradescantia sporocytes in water and described " spherical droplets 

 of a strongly refractive substance" formed by a "slow coagulation of 

 albuminoids in the cell contents" (see p. 431). They were more ade- 

 quately described a quarter of a century later by the investigators named 

 in the preceding paragraph. Waldeyer (1888) gave them their name. 

 The longitudinal splitting of the prophasic chromosomal threads was 

 discovered by Flemming in 1879; shortly thereafter (1884) van Beneden 

 and Heuser showed in animals and plants, respectively, that the daughter 

 halves pass to opposite poles in the anaphase. That they maintain their 

 individuality through the nuclear cycle was maintained by van Beneden 

 (1883), Rabl (1885), and Boveri (1887 et seq.). Drawings of the achro- 

 matic figure were published by Kowalevsky (1871) and Fol (1873), but 

 Biitschli (1875) was the first to describe it in detail. 



The reduction of the number of chromosomes was discovered by van 

 Beneden (1883), who announced that the nuclei of the egg and sperma- 

 tozoon of Ascaris each contain one half the number found in somatic 

 cells. Strasburger (1888) showed that in angiosperms the number of 

 chromosomes in the egg and male nuclei is fixed by a reduction occurring 

 in the megasporocyte and microsporocyte, respectively. This was at once 

 confirmed by Guignard (1889, 1891). E. Overton (1893) found that the 

 female gametophyte cells in a cycad, Ceratozamia, have half the number 

 present in the cells of the sporophyte. He further suggested that reduc- 

 tion probably occurs in the sporocytes in mosses and ferns. In a liver- 

 wort, Pallavicinia, Farmer (1894) found the gametophyte cells to have 

 four chromosomes and the sporophyte cells eight. That Overton's 

 theory of reduction in the sporocytes of bryophytes and pteridophytes 

 was correct was demonstrated by Strasburger (1894), who postulated the 

 occurrence of a periodic reduction in the number of chromosomes in all 

 organisms reproducing sexually. 



How the reduction is accomplished was not at first apparent. The 

 formation of functionless polar bodies by animal oocytes suggested that 

 the change in number is brought about by the simple casting out of half 

 the chromosomes during the development of the reproductive cells. This 

 view proved to be incorrect when it was shown (1890-1893) by Henking, 

 Riickert, Haecker, vom Rath, and others that the double chromosomes 

 appearing in the reduced number in the first meiotic mitosis are 

 really pairs of chromosomes. These bivalent pairs were seen to arise 

 by a synapsis of chromosomes two by two, the members of each pair then 

 disjoining in one of the meiotic divisions. Thus it became evident that 

 reduction is effected by a redistribution of the chromosomes to different 

 nuclei and cells and that the degeneration of the polar bodies is a phe- 

 nomenon without any general significance in meiosis. Studies on meiosis 

 in many groups of organisms were soon undertaken, and no department 

 of cytology has witnessed the development of a larger or more contro- 



