of I he Fishery Board for Scotland. 
295 
ment from the normal, while the vesicles in the first, both in their mode of 
origin, shape and grouping, diverged widely from the regular and irregular 
types of segmentation. 
In order to arrive at the order of development, it is impossible always 
to have the oosphere under observation, so the following account of the 
segmentation of Pecten opercularis, L., while in some instances recording 
the successive stages of development in one oosphere, is built up from 
examination of many specimens, confirmed by parallel phenomena re- 
peatedly exhibited by other oospheres. 
After fertilisation and the expulsion of the polar globule, there is no 
visible alteration in the oosphere for a few hours. When this period of 
rest has lapsed, the protoplasmic contents become much clearer at one 
place. This affords an index of the formative or animal pole. The 
oosphere ceases to be spherical, and becomes pear-shaped by the prolonga- 
tion of the clear end into a blunted or lobate process (PI. V. fig. 5). 
This lobate process is gradually drawn out (PI. Y. fig. 6), and its 
transparent point becomes darker, while a constriction appears at the 
base, and the oosphere is segmented into one macromere and one micromere, 
the micromere occupying the formative or animal pole (PI. V. fig. 7). 
The polar globule (p.g.) is usually found in the furrow between both cells 
(PI. V. fig. 8), but sometimes it is situated at the distal pole of the 
micromere (PI. V. fig. 7), as Moebius * has observed in the oyster. 
Neither at this nor at any subsequent stage has anything been seen 
corresponding to the lenticular bodies, which Bobretzky f describes and 
figures for Nassa mutabilis, and Wilson J for Mytilus edulis. 
The micromere i s at first sharply marked off from the macromere, but 
in a few minutes the surface of contact between the two is increased, and 
they are flattened against each other. The macromere becomes clearer 
near the furrow between it and the micromere, and a second micromere is 
budded off from it (PI. V. fig. 9). The contact surfaces of the micro- 
meral cells are flattened, and the appearance of the oosphere is as in 
fig. 9. Although at this point the micromeres generally subdivide, yet 
the macromere may again become active, when a third micromere is formed 
at its expense (PI. V. figs. 10 and 11). Whether, however, this last 
micromere is adjacent to that first formed or to the second one, was not 
observed. The micromeres are rounded on the peripheral part of the 
oosphere, but became flattened against the micromere. 
While these changes were going on, after the extrusion of the polar 
globule, no nucleus was plainly visible either in the ovum or in the 
macromere or two micromeres, but when the oosphere had reached the 
four-celled stage the segmentation nuclei were manifest. In fig. 10 the 
micromeres are flattened against the macromere, but in fig. 11 they are 
more rounded, and the polar globule is seen projecting beyond the middle 
micromere. The micromeres are now active, and each subdivides into two. 
The subdivisions arising from the oosphere with two micromeres, and that 
with three micromeres give rise to oospheres of one macromere and four 
micromeres (PI. V. fig. 12), and of one macromere and six micromeres 
respectively (PI. V. figs. 13 and 14). In some cases the area of 
attachment of the micromeres and macromere is greater than in others. 
This corresponds with the differences exhibited in figs. 13, 14, 15, 16, 
and 17. Where the area of attachment is greatest, the oosphere shows 
polar flattening ; where the micromeres are more crowded round at the 
formative pole the oosphere is elongated in the direction of the polar 
* Die Austcr mid die Austernwirthschaft, 1877. 
t Vide Balfour, Comparative Embryology, vol. i. p. 292. 
X Loc. cit., p. 251. 
