Later Spermatogonial of Fasciola hepatica. 75 
last spermatogonial divisions, a conjugation of chromosomes takes place 
in the telophase, and that twelve fused pairs of chromosomes go to 
form the resting nucleus of the spermatocyte I. cells. 
Before the resting nucleus stage is reached the mass of eight cells has 
lost its connection with the wall, and lies in the cavity of the gonad. 
PREPARATION FOR First MegIoTIC DIVISION. 
Signs of approaching division are to be found in the gradually increasing 
size of the nucleus. The chromatin becomes more distinct and retentive 
of stain, and more abundant, round the plasmosome (Fig. 23). In the rest 
of the nucleus it is present in little beads which do not stain so deeply as 
the blocks round about the plasmosome. 
The beads then form leptotene threads, but these are imperfect at this 
stage. 
Almost simultaneously with their appearance a parallel conjugation of 
the earliest formed threads begins, so that it is difficult to obtain zygotene 
views which show twelve pairs of univalent chromosomes. It is possible 
in one nucleus to see a certain number of conjugating chromosomes, while at 
another focus the same nucleus has all the appearance of one that has just 
emerged from the resting condition. Hence while the cumulative evidence 
obtained from a study of many spermatocyte I. nuclei leaves little doubt 
that a parallel conjugation of the chromosomes takes place, the material is 
not productive of diagrammatic figures. The pachytene stage is very 
common (Fig. 24), twelve completely fused bivalents being of frequent 
occurrence (Fig. 25). 
The chromosomes then seem to lose their staining capacity and to assume 
a beaded appearance (Figs. 26, 27), indications of their bivalent character 
being sometimes evident. Synizesis occurs at about this stage. The chromo- 
somes escaping from the synizetic mass are sometimes pale staining and 
somewhat beaded in character (Fig. 29), sometimes, however, they have begun 
to thicken and shorten preparatory to the onset of late prophase (Fig. 30). 
The strepsitene stage finishes simultaneously with the loosening of the 
definitive chromosomes from the synizetic mass (Fig. 30), the bivalent rings, 
crosses, loops, to the number of twelve, being the result (Figs. 31-33). 
In the metaphase of the first maturation division a great contraction of 
the chromosomes takes place. A well-formed spindle with centrosomes is 
present—equatorial plates giving counts of twelve chromosomes (Figs. 34, 35). 
The univalents then separate off to the daughter cells, and two nuclei of 
spermatocyte II. value result. The remaining history of spermatogenesis 
of Fasciola resembles very closely that described for Dicrocoelium. 
