296 
Part III. — Eighth Annual Report 
diameter. A similar change is manifest in the direction of the junction 
lines running between the macromere and micromeres. In figs. 15, 16, 
and 17 segmentation has proceeded at the expense of the micromeres 
previously formed; in fig. 17 the macromere appears more or less helmet- 
shaped, in fig. 15 it is nearly circular, while in fig. 16 the macromere is 
flattened at the side next to the micromeral segments, while it is rounded 
at the nutritive pole. Fig. 15, therefore, shows that the micromeres have 
spread over a greater proportion of the surface of the macromere than in 
cither fig. 16 or fig. 17. The tendency of the micromeres at each new 
subdivision is to extend themselves along the surface of the macromere, 
and envelope it. In fig. 17 (PI. VI.) the clear hyaline membrane which 
is also seen to envelope some of the ripe unfertilised ova is shown. 
As segmentation proceeds the spherical shape is lost, and just before 
the subdivision of the macromere, which is almost invested by the 
numerous micromeres, the oosphere is hat-shaped, with the macromeres 
projecting from the under surface (PI. V I. fig. 18). 
The oosphere is now composed of one macromere and numerous 
micromeres. The macromere begins to segment, the first division result- 
ing in the formation of two equal cells (PI. VI. fig. 19). These two 
cells are seen through the micromeral segments in fig. 19, and the appear- 
ance of the oosphere in side view is as represented at fig. 20. The 
micromeres which form the ectoderm have in great measure enveloped the 
two macromeres, which are the first two cells of the ectoderm. If fig. 19 
be compared with the same stage as is represented by Horst* (fig. 6), the 
likeness between the European oyster at this stage and fig. 19 will be 
manifest. The two macromeres become subdivided, and the oosphere is as 
drawn in PI. VI. fig. 21, where the four macromeres are visible through the 
enveloping ectoderm. In figs. 19 and 21 the margin of the oosphere, or 
periphery, exhibits a somewhat crenate outline, which is also seen in the 
two succeeding stages. "While the cells of entoderm continue to divide 
(PI. VI. figs. 22 and 23), they become completely closed in by the 
ectodermic layer. The oosphere has now a covering of cilia as represented 
in fig. 22, and the embryo performs a rotary motion, circling on its own 
axis, sometimes without moving away from the field of vision of the 
microscope, at other times making excursions out of the field. Fig. 22 
was drawn during a period of rest, which, however, is always of short 
duration. Fig. 23 shows at least eight cells of entoderm encircled by the 
ectoderm with two cells at the blastopore, and is the first stage at which 
the gastrula with macromeral cells as guard cells was seen. A gastrula 
fixed and stained showing the segmentation nuclei, is represented at fig. 
24. Here also the surface continuity is interrupted at one point, bp, the 
blastopore. The embryo (PI. VII. fig. 25) is now composed of a series of 
clearer peripheral cells clothed with cilia and darker central cells. By 
leaving the ciliated embryo in a small quantity of sea water in a watch 
glass till the water was reduced by evaporation, I found that the 
embryo became cleared up (PI. VII. fig. 26). Numerous small clear 
vesicles as well as granular particles could be seen all over the surface of 
the embryo, and in fig. 26 two cells projecting beyond the periphery were 
seen. Whether these projecting cells have any significance, further 
observations on other specimens did not throw any light on the subject. 
The next significant advance in the development of the embryo is the 
appearance of a recurved-like furrow (PI. VII. fig. 27) proceeding from a 
smaller depression, sp, on the rounded surface of the embryo. This is 
indicated in fig. 27 in a living and actively moving embryo, and in fig. 28, 
which was fixed and stained. In the latter it is seen to occupy a position 
* De ontwikkelingsgeschiedenis van de Oester {Ostrea edulis), 1884. 
/ 
