COMPLETION OF MEIOSIS 119 



One constant distinction is observed at the prophase from the 

 normal behaviour at mitosis : the two chromatids are held together 

 only at the centromere. Elsewhere they actually seem to repel 

 one another until metaphase. This is perhaps to be associated with 

 the unique property of the interphase period between the two 

 meiotic divisions, viz., that there is no division of the chromosomes, 

 that the associated chromatids are derived partly from association 

 with other chromatids, so that they have undergone spiralisation 

 separately. The theoretical significance of this will be considered 

 later (Ch. XII). 



(x) The Second Division. The second division follows the 

 course of an ordinary mitosis. The chromatids momentarily come 

 together for part or all of their length. Where they are globular 

 they lie in the axis of the spindle. They are pulled apart at their 

 centromeres which here even more clearly than at mitosis are alone 

 concerned in their separation. Four daughter-nuclei are formed, 

 each with half the number of chromosomes of the mother-nucleus. 

 Meiosis is complete. 



4. MEIOSIS IN POLYPLOIDS 



(i) Prophase. It is known from the fact that chromosomes may 

 pair in parthenogenetic organisms {q.v.), that their origin, whether 

 maternal or paternal, has no connection with pairing. Therefore 

 in polyploids having several identical chromosomes of each type 

 instead of two, we must expect pairing to occur indifferently 

 amongst all the chromosomes of each type. But there are several 

 different ways in which we might suppose this to come about. 

 For example, at zygotene the threads might act as units and associate 

 in pairs throughout their length, as they do in diploids. Or they 

 might associate in threes or fours, according to the number present 

 of each kind, to give a triple or quadruple thread at pachytene. 

 Neither of these surmises holds good. 



In both triploids (Tiilipa, Fritillaria, Zea Mays and Hyacinthtis), 

 and tetraploids (Hyacinthus, D., 1929 6 ; Allium, Levan, 1935 b), 

 the chromosomes come together in pairs. In a triploid, therefore, 

 one thread is left out of association, while in a tetraploid two separate 

 pairs are formed. But different pairs are formed at different points. 



