22 
necessary in each case. A few were fixed with Flemming’s solution, 
but it was found difficult to stain satisfactorily after the use of this 
fixative. 
The small size of the cellular elements in the testis prevents Daph- 
nia pulex from being a favourable subject for a detailed study of the 
early prophase of the meiotic division and of syndesis, so the work was 
chiefly confined to a determination of the number of chromosomes in 
different tissues, and in the various generations of cells in the testis. 
The somatic number of chromosomes in this species is given by Kühn 
as 7—10, probably 8. 
_ The gonad of the male Daphnia pulex is paired, and lies right and 
eft of the alimentary canal. In the younger specimens each testis is 
solid, and consists of the relatively large primordial germ cells, the 
nuclei of which are commonly in the resting condition. Examples of 
mitoses among these cells are not easy to obtain — the gonad becoming 
very active only when the individual is three days old. Daphnia of from 
4 to 5 days old are most useful for showing all stages in the spermato- 
genesis. In such specimens each testis consists of an elongated, lobu- 
lated sac pointed at the anterior end, and passes insensibly into the vas 
deferens. The sac is incompletely and irregularly subdivided into com- 
partments by the presence, in the walls of the gonad at irregular inter- 
vals, of large cells probably somatic in character. The developing sper- 
matogonial elements are found in clusters in the peripheral parts of the 
sac; in the gradually appearing central lumen are ripe spermatozoa. In 
a creature 6 days old the whole gonad is filled with ripe spermatozoa 
which lie in a matrix which stains deeply with eosin, the walls of the sac 
being by this time exceedingly thin, and the large somatic cells having 
almost entirely disappeared. The chromosomes are most easily counted 
in prophase, when it is quite possible to show that the number of chromo- 
somes in the spermatogonial cells is the same as in those of the tissues 
of the female, and lies between 8 and 10, fig. 1, a. a’. 
In the metaphase of the spermatogonial and meiotic divisions the 
chromosomes are so close together that, though equatorial plates are 
exceedingly numerous in the sections, and the spindle apparatus and 
centrosome are well organised and quite typical, yet these do not form 
favourable objects for counting chromosomes, fig. 1, b. (Cf. equatorial 
plate from a cell in the intestine, c). 
A well marked synizesis stage occurs in the early prophases of the 
first meiotic division, and a well defined nucleolus, most easily seen in 
sections stained in Ehrlich, is present until the chromosomes are well 
condensed. Late prophases are characterized (fig. 1, d.) by the presence 
of the haploid number of chromosomes (i. e. 4 to 5). These are larger 
