18 J. E. WODSEDALEK. 



one-half takes place. Before this actual reduction, there is usually 

 a so-called pseudo-reduction, in which the chromosomes unite in 

 pairs so that when the cell is ready for division, although only 

 half of the regular number of chromosomes appear, each is really 

 double or bivalent and equivalent to two of the single or univalent 

 type. 



In this hybrid, as was suggested before, it may be supposed 

 that in the somatic cells and in the spermatogonia, the chro- 

 mosomes from the paternal and maternal animals lie side by 

 side and carry on the customary functions of the cells; but when 

 it comes to an actual fusion of the chromosomes to form the 

 bivalent type necessary for reduction, the incompatibility of the 

 two plasmas renders the pairing difficult and incomplete, or 

 prevents it entirely. 



In the horse pairing or pseudo-reduction takes place during 

 the period of synizesis. This period is characterized by massing 

 of the chromatin threads in the center of the nucleus, and later 

 the nuclear wall expands and the entire mass passes to one side 

 of the nucleus leaving a large clear area in the remaining portion 

 (Wodsedalek, '14). This condition is much the same as in the 

 pig (Wodsedalek, '13), except that in that animal the nucleoli 

 were invariably found within the mass of threads and in a position 

 nearest to the nuclear wall; while in the horse the nucleoli are 

 almost invariable within, or next to the clear area. Shortly 

 after the collapse of the chromatin material, the threads pair 

 and appear in apparently half the original number and twice 

 as thick. Later they expand and again occupy the entire contents 

 of the nucleus. Then follows the period of growth, and eventu- 

 ally the threads break up into the bivalent chromosomes which 

 appear in half the original number found in the spermatogonia, 

 plus the accessory. 



The pairing of the various components, in the horse, takes 

 place simultaneously, and when the spireme appears it possesses 

 a fairly uniform diameter throughout. This fact indicates 

 clearly that each portion of the spireme is of a bivalent nature, 

 for should some parts remain unpaired or univalent in nature, 

 such parts could be readily discerned by the sudden and notice- 

 able decrease in their diameters. No pairing of parts in the horse 



