MEIOSIS 265 



each of the tetrads seen in the late prophase of the first mitosis. Cyto- 

 kinesis soon ensues, giving in typical cases four cells (spermatids; egg 

 and polocytes; spores), each with the reduced, or gametic, number of 

 chromosomes in its nucleus. 



In certain cases evidence has been brought forward to show that the 

 chromonema in each chromatid at the close of the second meiotic mitosis 

 is already split "in preparation for" the first postmeiotic division. The 

 time of origin of this split is difficult to determine. In Gasteria it is 

 clearly seen in the early prophase of the second meiotic mitosis (Fig. 

 155, d), and its presence in the anaphase of the first mitosis is suggested. 

 In Tradescantia it can be seen at this stage, while in Zea it has been 

 observed in diakinesis.^^ This accords with the evidence that in large 

 somatic chromosomes the chromonema is longitudinally doubled at least 

 one mitotic cycle in advance of the anaphase in which separation is to 

 occur along the plane so marked out (c/. p. 137). Whether or not this is 

 true of the last premeiotic mitosis in forms with wholly typical meiosis is 

 still a debated question. We shall revert to this point later in the chapter. 



Disjunctional and Equational Division. — It has now been shown that 

 each tetrad chromosome appearing in the late prophase of the first meiotic 

 mitosis is formed by the synaptic union of two homologous chromosomes 

 which in turn are longitudinally split ; and, further, that the four chroma- 

 tids composing the tetrad at late prophase are distributed to four nuclei 

 (and cells) by the two meiotic mitoses. From this it would appear that 

 one of the mitoses must be disjunctional, in that it separates synaptic 

 mates, while the other is equational, since it separates sister chromatids. 

 Which mitosis is to be regarded as disjunctional (reductional in the strict 

 sense) and which as equational? 



The four chromatids composing a tetrad are ordinarily similar to one 

 another in appearance when fully condensed, so that it is difficult or 

 impossible to determine by direct observation whether the separation in 

 the first anaphase is along the synaptic or along the equational plane. 

 It has, however, been found in some cases, notably in certain insects, 

 that the synaptic mates are sometimes unlike in size and hence form 

 " heteromorphic " tetrads with two large and two small chromatids. 

 Observations on the behavior of distinguishable chromosomes of this kind 

 have made it possible to show that a given tetrad may divide disjunction- 

 ally in I in some cells but equationally in others, at least in the hetero- 

 moipiiic region. In certain species there is nevertheless a strong tendency 

 to behave in one way rather than the other. ^^ Furthermore, since the 



" W. R. Taylor (1931) on Gasteria, Nebel (1932) on Tradescantia, McClintock 

 (unpubl.) on Zea. 



1^ Wenrich (1916) on Phrynotettix, McClung (1928a) on Mecostethus, Carothers 

 (1931) on T rimer otr opts, Mecostethus, and Amphitornis. The behavior of hetero- 

 morphic sex-chromosome tetrads, summarized by Wilson (1925, p. 753), tends to be 

 uniform in a given species. 



