252 INTRODUCTION TO CYTOLOGY 



ences between meiosis and mitosis of the somatic type. These points are 

 illustrated in Fig. 149. 



In the division of a somatic nucleus each and every chromosome of the 

 complement divides longitudinally into two halves which come to lie in the 

 two daughter nuclei. Both of these nuclei are therefore like the original 

 nucleus in containing equivalent derivatives of all the chromosomes; 

 hence somatic mitosis is said to be equational. 



In the prophase of the first meiotic mitosis the two members of each 

 pair of homologous chromosomes approach each other and come into 

 intimate association; this is known as synapsis. As a result of synapsis 

 the nucleus contains bivalent chromosomes in the reduced number. ^ At 

 some period during the prophase it becomes evident that each of the 

 members of a synaptic pair is double as the result of a longitudinal split- 

 ting. Hence each bivalent appears not simply double, but quadruple: 

 it is a tetrad chromosome. The four members of each tetrad, two of them 

 derived from one synaptic mate and two of them from the other, are 

 known as chromatids.^ At the close of the first meiotic prophase, there- 

 fore, the meiocyte nucleus contains the reduced number of tetrads. 



In the two meiotic mitoses the four chromatids of each and every 

 tetrad are distributed (with or without certain alterations) to the four 

 resulting nuclei. When the tetrads are oriented in the spindle as shown in 

 the diagram, the two paternal chromatids are separated from the two 

 maternal ones'* with which they have been in synapsis; this is known as 

 disjunction, or reduction in the stricter sense. Each two associated 

 chromatids passing to each pole constitute a dyad. When disjunction 

 occurs in the first mitosis the second mitosis is obviously equational in 

 character, since dyads which are then separated into single members are 

 sister chromatids formed by longitudinal division. 



In some cases it is known that tetrads may be so constituted and 

 oriented in the spindle that they divide (at least in part; see p. 265) 

 along the plane of splitting in the first mitosis and consequently along 

 the synaptic plane (disjunctionally) in the second. Both types of division 

 may occur in the same cell, so that each of the two meiotic mitoses may be 

 both disjunctional (for some elements in the chromosomes) and equational 



2 This is often called "pseudoreduction." Bivalent chromosomes are sometimes 

 called gemini (= twins). "Homologous" chromosomes in the complement are those 

 which correspond in function (see p. 272). 



^ It is customary, though somewhat awkward, to apply the term "chromosome" 

 rather indiscriminately to single chromosomes, split chromosomes, univalents, biva- 

 lents, multivalents, and tetrads. When the two halves (chromatids) of a split 

 chromosome or the four chromatids of a tetrad separate from one another, each is 

 then referred to as a "chromosome." 



* It is convenient to designate the chromatids in this way because the synaptic 

 mates are ordinarily derived from the male and female gametes. It will be shown, 

 further on, that this is not always the case, and that parental derivation is not the 

 essential feature of "homology." 



