The Sexual Phases of Myzostoma. 
233 
epithelium (Fig. 16 o.m). In this figure one of the oogonia is seen 
dividing at oog ^ the spindle being seen from the pole. Ventral 
to the intestinal branches numerous spermatogonia are dividing [spg.p] 
and the section also shows a mass of spermatids with nuclei elongat- 
ing to form the heads of the spermatozoa [sp]. In the other sections 
of the same series there are many mature spermatozoa. 
Stage 6. 0.33 — 0.55 mm long. The intestinal and coelomic 
diverticula increase still further in length and their lateral ends 
begin to form secondary ramifications. The ultimate tips of these 
branches end just above the insertions of the parapodial setae. The 
triplet-cells, i. e. the migrating oocytes with their accessory cells^ 
still occur in the ramifications of the body-cavity. Some of these 
have attached themselves to the dorsal wall of the body-cavity and 
have begun to grow into ova; but the number of growing oocytes is 
limited as yet. The number of mature spermatozoa, however, has 
increased considerably and many have already found their way to 
the vasa deferentia. 
Stage 7. 0.6 mm long (Fig. 3). By this time the »uterus« 
or that portion of the body-cavity which lies in the mid-dorsal line, 
has been formed by the fusion of the circumovarial body-cavity ex- 
tending inwards from either side. Testicular follicles have made 
their appearance in the parenchyma over the body-cavity. The 
vesiculae seminales are full of ripe spermatozoa, and the animal 
probably discharges them into the water from time to time. Many 
of the oocytes have attached themselves to the dorsal peritoneum of 
the body-cavity and its ramifications. Some of these oocytes have 
already attained about a fifth or fourth of their adult volume. 
Stage 8. 1mm long. In this stage the ramifications of the 
intestine and body-cavity have extended themselves still further 
into the flattened lateral edges of the body and have undergone 
further subdivision into minute branches. The number of ova 
attached to the dorsal walls of the body-cavity and its ramifications 
has greatly increased, but none of them have as yet reached maturity. 
The ovaries themselves show an indication of division on either side 
into two masses. This is readily seen in horizontal sections like 
Fig. 2 [ovM ov.p). On one side of the figure the ovary is trilobed, but 
the middle and posterior lobes are together evidently the equivalent 
of the posterior ovarian lobe [ov.p) on the opposite side of the body. 
Stage 9. 1.75 — 2.5 mm long. The ultimate ramifications 
of the intestine and superjacent body-cavity have extended them- 
