MEIOSIS 



263 



extent to which the telophasic transformation is carried varies greatly 

 in different cases. Sometimes what are essentially metabolic nuclei 

 are formed, but probably more often the changes do not proceed far 

 enough to render the individual dyads indistinguishable. In extreme 

 cases, notably certain animal oocytes (Fig. 154), the second mitosis 

 follows so closely upon the first that the dyads at the close of I, imme- 

 diately and without any structural change, take their places in a spindle 

 newly formed for //. The interval between the two meiotic mitoses is 

 known as interkinesis. Ordinarily the two nuclei at this stage come to be 



Fig. 154. — Meiosis and syngamy in Ascaris. A, spermatozoon about to enter egg. 

 B, spermatozoon inside; first meiotic division in progress. C, first mitosis completed; 

 first polar body budded off. D, second meiotic division, forming second polar body; 

 nucleus of spermatozoon below. E, meeting of male and female pronuclei, each with 2 

 chromosomes. F, first cleavage mitosis, showing 2 paternal and 2 maternal chromosomes. 

 (After O. Hertwig.) 



fairly large, and within them the chromatids appear as somewhat slender, 

 crooked threads associated in dyads at their spindle-attachment regions 

 (Figs. 150, 155). Anastomoses may be more or less evident, depending 

 on the degree of telophasic transformation. 



Cytokinesis follows the completion of division I in many meiocytes. 

 In animals the primary oocyte is thus divided into a secondary oocyte and 

 a polocyte, and the primary spermatocyte becomes two secondary sperma- 

 tocytes. In the sporocytes of higher plants cytokinesis may follow divi- 

 sion /, but in many cases it is delayed until after //, when the sporocyte 

 is divided simultaneously into four spores (p. 167). In the meiocytes of 

 lower plants various conditions are known. 



