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W. M. Wheeler 
reprodiietive elemeuts liave as yet advauced beyond this point. The 
ovaries are in the same condition as in stage 1 ; they are mere flat- 
tened acciimulations of young oogonia, or modified peritoneal epi- 
thelial cells on either side of the intestine. 
Stage 3. 0.3 — 0.35 mm long (PI. 11 Fig. 25). The in- 
testinal diverticiila extend out on either side beyond the insertions 
of the parapodia, but are still quite simple. The body-cavities on 
either side above these diverticula are only just fusing in some 
specimens in the median line above the intestine to form the »uterus«. 
The thickened portion of the body ventral to the intestine and its 
branches is full of proliferating male elements ■ — many actively dividing 
spermatogonia and spermatocytes and a goodly number of mature 
spermatozoa. The ovaries are somewhat larger than in the preceding 
stages, but the two kinds of cells which are so characteristic of these 
Organs in a later stage are not yet clearly seen, although the be- 
ginning of such a differentiation is perhaps already indicated by a 
difference in the size of the nuclei. Myzostomes in this stage 
are thus functionally male. 
Stage 4. 0.375 mm long. The intestinal diverticula are 
developing further ramifications. The body-cavity has been obliterated 
with the excepfcion of a very small space surrounding the ovaries. 
These are somewhat larger, but otherwise in about the same condition 
of development as in the preceding stage. The total volume of the 
testicular follicles has increased considerably, and there are a great 
number of mature spermatozoa. 
Stage 5. 0.5 — 0.8 mm long. The body-cavity has become 
larger about the ovaries, but is not apparent elsewhere in the sections. 
The ovaries have increased in volume and the two kinds of cells,. 
the oogonia and accessory cells, stand out clearly; the latter often 
staining very deeply in this stage. Instead of being smooth as in 
the preceding stages, the surface of the ovary becomes somewhat 
irregulär and small cavities begin to invade the hitherto solid tissue 
of the Organ. These cavities are large and distinct in specimens 
0.8 mm long. 
The greater portion of the Myzostome’s body is now filled out 
with the testicular cells in all stages of division and metamorphosis 
into the spermatozoa. There are mature spermatozoa in abundance, 
both in the recesses of the ventral parenchyma and in the cavities 
of the vesiculae seminales. A few testicular follicles have also made 
their appearance in the parenchyma dorsal to the intestinal rami- 
