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. Riickert (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. 61 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.6dande). 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 “ving type.’ Almost every primary sporocyte contains 
from one to several (8 or 9) ring forms in different stages. In 
