II MEIOSIS IN TOMOPTERIS 31 



The fully formed spermatocytes I. — that is to say, just before the 

 meiotic mitosis takes place — are considerably larger than the later 

 generations of spermatogonia. Hence the interval between the last 

 spermatogonial mitosis and the meiotic division is called the growth 

 ■period. It is during this period that the changes of the long drawn-out 

 meiotic prophase are found. 



In the female the growth period is much more conspicuous than in 

 the male, since it is during this period that the yolk, which is so abundant 

 in most macrogametes, is deposited, and hence the primary oocyte (or 

 ovocyte) is much larger than the primary spermatocyte. Meiosis I., 

 instead of resulting in two similar oocytes II., is followed by a very 

 unequal cell division, resulting in one oocyte II., very nearly as large as 

 the parent cell, and a minute cell, the " first polar body." Meiosis II. 

 is followed by a similarly unequal cell division, resulting in the mature 

 ovum and " second polar body." 



A. MEIOSIS IN THE MALE 



The remarkable interest of the meiotic processes has resulted in a 

 great deal of attention being paid to this phase, and it has been investi- 

 gated in many species both of animals and plants. Unfortunately, the 

 difficulties of observation and interpretation are great, and have resulted 

 in a corresponding diversity of views as to the precise mode by which the 

 halving of the chromosome number is effected. 



In the first place we wiU describe the process as it occurs in the male 

 of the Polychaete worm Tomopteris (as described by A. and K.E. Schreiner, 

 1906 a), after which the more important variations on this scheme, or 

 different interpretations thereof, will be discussed. 



(i) Meiosis in Tomopteris onisciformis (Fig. 14) 



The spermatogonial or pre-meiotic divisions present no special 

 differences from the somatic mitoses. The number of chromosomes is 

 eighteen. (It wiU of course be understood that the figures, being depic- 

 tions of actual sections, show only such chromosomes, or portions thereof, 

 as occur in the sections. ) We may, therefore, begin our detailed description 

 with the last pre-meiotic telophase (Fig. 14, A, B). The daughter nuclei 

 reconstructed from this telophase are the primary spennatocytes. In 

 them the chromatin is arranged in a fine network, in which, however, 

 chromosome areas are said to be discernible in the form of parallel bands 

 along which the chromatin is more densely aggregated. A. and K. E. 

 Schreiner believe that by means of these bands the chromosomes of the 

 last pre-meiotic telophase can be traced continuously into the chromosomes 

 of prophase I. A conclusion of this nature, involving as it does the 

 negative demonstration that at no time between the two phases do the 



