106 



2. Period of tetrad formation ; pseudo-reduction. 



In the case of animals when the spireme thread breaks up into 

 segments destined to form tetrads, the number of these segments 

 is, in general, half the number of chromosomes in the somatic 

 cells. There is a reduction in number of chromatin masses, but 

 the nucleus still contains all the chromatin it held at first, so that 

 actual reduction has not yet taken place. Ruckert (1894) has ac- 

 cordingly proposed the expressive term " pseudo-reduction " for 

 this preliminary halving of the number of chromosomes. 



Pteris forms no exception to this rule. The double spireme 

 breaks up into short and well defined chromatin segments (Fig. 

 5 a) each of which gives rise to a tetrad. The number of these seg- 

 ments is difficult to determine ; in several cases I counted about 

 sixty. This is about half of the number in somatic cells where, as 

 nearly as I can make out, there are between one hundred and 

 twenty and one hundred and thirty chromosomes. It is an inter- 

 esting fact that the process of tetrad formation is subject to some 

 variation and does not, apparently, conform exclusively to any 

 one type. This conclusion is based upon the following facts. 

 The spireme segments are, from the beginning, invariably double 

 (Fig. 5 a). The same nuclei contain various modifications of the 

 double segment. Some of them are split in the center while the 

 ends remain connected, giving rise to ring forms (Figs. 4, 5, 19 c). 

 In some there is no separation at all, in others the ends separate, 

 giving rise to "cross" forms (Fig. 6 1 and Fig. 19 a) and in still 

 others one half the segment may slide along on the other half till 

 the ends are no longer contiguous (Fig. 6 d and e). There may 

 be still further modifications of the double segment in the same 

 nucleus (Fig. 5 x). In none of the nuclei which I have examined 

 does any of these types predominate ; and from their various and 

 diverse shapes it is impossible to regard them as developmental 

 stages of a single type. I am forced, therefore, to the conclusion 

 that, in these ferns, tetrads may be formed in a variety of ways. 

 The various methods can be grouped into three types, which I 

 will describe separately as (a) the "ring type;" (b) the "rod 

 type ;" and (c) the " cross type." 



a. The " ring type." Almost every primary sporocyte contains 

 from one to several (8 or 9) ring forms in different stages. In 



