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Part III. — Eighth Annual Report 
their velar ends. The anterior adductor, ad, is well developed in front 
of the velum, and at the upper angles of the valves near the hinge line are a 
number of small oval cells. The indications of the mantle are shown both 
posteriorly and anteriorly. 
Although this was the most advanced larva which I succeeded in rearing 
in the glass aquaria, I obtained by the tow-net free swimming veliger 
larvre, with distinct heart, gill papillae, and other organs. 
EXPLANATION OF THE PLATES. 
Fig. 1. Egg of the Scallop (Pecten opercularis, L.), showing the nucleus and nucleolus 
within the vitellus. Zeiss Oc. 3, lens D. 
Fig. 2. Spermatozoon. Zeiss Oct. 4, Seibert T V im. lens. 
Fig. 3. Spermatoblasts showing spermatozoa with heads imbedded and free tails. 
Fig. 4. Ovum showing polar globule on circumference ; the nucleus has become 
invisible. Zeiss Oc. 3, lens D. 
Fig. 5. Oosphere, with animal pole drawn out. 
Fig. 6. Oosphere, with animal pole more drawn out. 
Fig. 7. First stage of segmentation; the oosphere has become divided into a macromere 
and micromere, the polar globule being visible near distal pole of the 
micromere. Zeiss Oc. 4, lens A, doubled. 
Fig. 8. First stage of segmentation, the macromere and micromere are flattened 
against each, and the polar globule is seen in the furrow to the right. 
Zeiss Oc. 4, lens D — one-third less. 
Fig. 9. Second stage of segmentation ; oosphere of one macromere and two micromeres. 
Zeiss Oc. 4, lens A, doubled. 
Fig. 10. Oosphere of one macromere and two micromeres ; the nuclei of the segments 
are visible. 
Fig. 11. Oosphere of same stage as preceding, with micromeres more rounded and 
polar globule seen opposite middle micromere. 
Fig. 12. Oosphere of one macromere and four micromeres, with polar globule between 
macromere and micromere. 
Fig. 13. Oosphere of one macromere and six micromeres. Figs. 9-13. Zeiss Oc. 4, 
lens A, doubled. 
Fig. 14. Oosphere of same stage as preceding, but not so flattened. Zeiss Oc. 4, lens 
D, one -half. 
Fig. 15. Oosphere of one macromere and numerous micromeres, the latter extending 
half-way round the former. 
Fig. 16. Oosphere of one macromere and numerous micromeres, the micromeres 
occupying about one-half the mass. 
Fig. 17. Oosphere of one macromere and numerous micromeres, the micromeres 
occupying less of the mass than in 15 or 16 ; a clear hyaline investment 
is seen surrounding the oosphere. Figs. 15-17. Zeiss Oc. 4, lens A, 
doubled. 
Fig. 18. Oosphere with the numerous micromeres enveloping the macromere. 
Fig. 19. Oosphere with the macromere segmented into two seen through the 
micromeres. 
Fig. 20. Oosphere of the same stage as preceding seen in profile. 
Fig. 21. Oosphere still more advanced where theft rst macromeres are subdivided. 
Fig. 22. Gastrula (ciliated), with the ectoderm and entoderm visible. 
Fig. 23. Gastrula with macromeres forming the entoderm, and showing two macromeral 
guard cells at side of blastopore, bp. Figs. 18-23. Zeiss Oc. 4, lens D. 
Fig. 24. Gastrula fixed and stained showing blastopore bp, and segments with nuclei. 
Zeiss Oc. 4, lens D. 
Fig. 25. Ciliated gastrula showing clearer peripheral edge and darker cells towards 
middle. 
Fig. 26. Ciliated gastrula showing clear vesicles ; cleared up by evaporation of the 
sea- water in the sun. Zeiss Oc. 4, lens D. 
Fig. 27. Embryo showing the depression, sh, of the preconchylian gland. 
Fig. 28. Embryo fixed and stained, snowing blastopore, bp, and depression for 
preconchylian gland, sh. 
Fig. 29. Embryo showing position of blastopore, bp ; depression of preconchylian 
gland, yl, and velum, v. 
Fig. 30. Embryo showing position of blastopore, bp; velum, v; and incipient shell, sh. 
Fig. 31. Irregular ciliated embryo, showing developing shell, sh, clear vesicles, and 
lobate process. 
