450 Miscellaneous. 
to be incorrect ; at least, in the Sepiole this membrane is furnished 
with a micropyle, and must therefore be considered a chorion. 
The ova of the Sepiole are completely transparent. Their de- 
velopment lasts from thirty-four to thirty-five days. The chorion 
undergoes modifications in proportion as the foetus is developed : it 
increases in size and its thickness diminishes ; moreover it changes 
its form, and, from being ovoid, becomes spherical towards the close 
of the development. 
The author distinguishes three periods in the embryogenic deve- 
lopment of the Sepiole: the first, which extends to the completion 
of the blastoderm, lasts ten days; the second, during which the 
organs appear, lasts only five days; and the third, during which the 
organs previously formed are gradually developed, lasts nineteen or 
twenty days, and terminates with the exclusion. 
First period.—The phenomenon of partial segmentation has not 
been studied from its commencement by the author. The youngest 
ova which he had under his hands already had the pointed pole of 
the vitellus covered with embryonal cells. Between this first rudi- 
ment of the blastoderm and the chorion some drops of protoplasm 
seem to correspond to the supposed directive cells (Richtungs- 
blischen) of the Gasteropods, Insects, &c. The nucleated cells of 
the blastoderm form at first a single layer; they seem to multiply 
by division. Towards the end of the second day, the blastoderm is 
extended so as to cover two-elevenths of the vitellus. 
On the third day the blastoderm divides into two superposed 
lamellz. This stratification seems to be the result of a transverse 
division of the primitive cells. The two lamelle are similar in thick- 
ness, but in each of them the thickness diminishes towards the 
margins of the blastoderm. 
In the latter part of this period the edges of the blastoderm gra- 
dually extend over the vitellus. They envelope the half of it by 
about the eighth day, and on the tenth they meet at the superior 
pole of the ovum, enveloping it entirely. The rapidity of the growth 
in the last two days is explained by the difference of thickness of the 
different parts of the blastoderm. In fact the lower region, corre- 
sponding to the future embryo, presents a much greater thickness 
than the upper part, which is to become the vitelline or umbilical 
vesicle. During the whole of this first period each of the lamelle 
of the blastoderm is formed only by a single layer of cells. These 
are capable of executing very marked amceboid movements. 
Second period.—From the commencement of this period the cells 
of the outer lamella of the superior part of the blastoderm become 
covered with vibratile cilia, the movement of which causes a rotation 
of the embryo. At the same time the blastoderm (especially the 
inner lamella) thickens in its lower region, and the foetus begins to 
be distinguished from the vitelline vesicle placed above it. Soon a 
thickening of the blastoderm with an oval outline makes its appear- 
ance on each side of the body, a little below the equator of the 
vitellus. These inflations, which belong essentially to the outer 
lamella, and which from the second day are very distinct, are the 
