MEIOSIS 255 



each nucleus of the quartet resulting from the meiotic divisions contains 

 but one member of each homologous chromosome pair, rather than two as 

 in the meiocyte nucleus. Although qualitative differences among the 

 four nuclei usually obtain as a result of certain dissimilarities in the 

 homologous chromosomes, such differences are not always a necessary 

 consequence of meiosis. 



The main points brought out in this section may be summarized provi- 

 sionally as follows : 



In a somatic mitosis each chro7nosome of the com'plement is divided 

 longitudinally, the halves passing to the daughter nuclei; hence these nuclei are 

 similar to each other and to the original nucleus: somatic mitosis is equational. 



In meiosis each chromosome enters into synapsis with its homologue and 

 also splits longitudinally; this gives a tetrad chromosome composed of 

 four chromatids. The four chromatids of every tetrad are distributed to 

 the four nuclei by the two meiotic mitoses. Each chromosome {or portion of 

 a chromosome) is thus disjoined from its homologue in one mitosis and 

 divided equationally in the other. 



Each of the quartet nuclei contains a monoploid chromosome set made up 

 of a longitudinal half of one member of each of the original homologous pairs. 

 The four nuclei differ qualitatively among themselves to the degree in which 

 the various chromosomes differ from their respective homologues, and they 

 furthermore differ from the original meiocyte nucleus in having only half as 

 many chromosomes. 



In the next chapter evidence will be presented to show that meiosis 

 involves essentially a change from the condition in which both members of 

 a pair of corresponding genetic units are present in the nucleus to the 

 condition characterized by the presence of but one. This change may be 

 accomplished for all of the units at one of the meiotic mitoses, or for some 

 units at the first mitosis and for others at the second. 



Detailed Description of Meiosis. — The literature pertaining to 

 meiosis is characterized by a sharp conflict of opinion regarding the struc- 

 tural alterations which take place, especially during the first prophase. 

 This divergence in interpretation has been both annoying and stimulating, 

 especially because important theoretical considerations have been 

 involved. During the past few years some of the long-debated questions 

 have received generally accepted answers, but other problems have arisen 

 in their places. On the whole, very substantial progress has been made in 

 this important field. 



In this section we shall describe the successive appearances presented 

 by the nuclear elements during the process of meiosis. The course of 

 events is naturally not precisely the same in all organisms, but the follow- 

 ing account is found to apply very widely, at least so far as essentials are 

 concerned. In order to avoid confusion it will still be necessary to post- 



