156 Proceedings of the Royal Society of Edinburgh. [Sess. 
of maturation. During the period of synapsis the chromation element of 
the nucleus consists of a number of fine filaments ; these assemble in a skein, 
out of which they emerge as a thick spireme which becomes reticulated. 
This thickened filament undergoes longitudinal division, which is only 
apparent as it is the separation of the fine filaments which were parallel in 
the previous stage. These fine filaments are probably the somatic chromo- 
somes, and are in reality “ chromosomes bivalents.” The mitosis of matura- 
tion is identical in both spermatocytes and oocytes. The first mitosis is 
heterotypical and the second homotypical, and it is the latter which affects 
the numerical reduction (only apparent at the prophase), and consequently 
Cyclops verifies the pre-reduction type. 
Farmer and Moore (8) point out that each heterotype chromosome is a 
bivalent structure, and their reduced number (half that of the somatic 
chromosomes) is due to the approximation and more or less intimate though 
temporary union of the equivalents of pairs of somatic chromosomes. The 
later stages of maturation in the oocyte can be studied from the papers of 
Victor Gregoire (10), Bolles Lee (3), Bryce and Van der Stricht (24). 
The theories as to the duality of the filaments in the maturation phases 
are as follows : — (a) There is first longitudinal division of the filaments, 
separation then union of pairs, forming a thick cord, which undergoes 
a second longitudinal division, (b) The filaments of the first division 
remain parallel by means of their achromatic connections, having a thick 
appearance as of a single cord, then reappear as separate cords when these 
connectious become lost, (c) In the synaptic stage two filaments unite and 
form a thick cord, then separate into parallel but distinct filaments. The 
third theory has the support of v. Winiwarter, owing to the observation 
that the concentration of the filaments in the synaptic stage is for the 
purpose of bringing different parts of the cord into contact, and by their 
union to form thick cords. When the chromatic segments are formed, the 
structure of the cord is modified, and their double constitution becomes 
visible, because the achromatic connections are relaxed. The synaptic 
stage is found in all animals, and the cords enter the skein singly. If the 
theory is true, the diminution of half the number of chromosomes at the 
time of the appearance of the first polar body is explained by union of the 
chromatin filament itself. The total number of segments will be equal to 
the typical number of chromosomes of the kind, but being united longitudin- 
ally two by two, their number is evidently diminished by half. Tetrads are 
formed by longitudinal division of each filament. The change which occurs 
in the nucleoli of the cell are referred to by most writers. Wilson has the 
view that there is one principal nucleolus and several accessory nucleoli. 
