Miscellaneous. 451 
rudiments of the eyes. At the same period there appears on one 
side of the ovum a small fold of the outer lamella, which speedily 
extends all round the embryo, and constitutes the rudiment of the 
mantle. On the second day, also, the mouth shows itself on the 
ventral surface as a depression in the form of a horseshoe. Then 
the branchie originate, and the first two pairs of arms and the 
organs of hearing. All these organs are formed chiefly at the ex- 
pense of the inner lamella, the outer lamella serving them only as a 
protective envelope. On the third day of this period, the rudiment 
of the mantle presents on the dorsal side a partial thickening of the 
outer lamella, corresponding to the point which will subsequently 
serve for the secretion of the os Sepia. 
At this period the embryo is constricted in its middle by a line of 
demarcation which divides it into two parts: the lower one is the 
foetus properly so called; the upper one includes the arms and the 
vitelline vesicle. The two lamelle of this vesicle separate from each 
other, although still united by very fine fibres, which are probably 
prolongations of the cells of the inner lamella. 
On the fourth day the anal tubercle and the rudiment of the 
siphon (infundibulum) make their appearance. This last is formed 
of two distinct bands, inclined 45° towards the equator and diverging 
from above downwards. These bands are thickenings of the inner 
lamella; the outer lamella simply covers them, without taking part 
in their formation. The intestinal canal, the acoustic sacs, the eyes, 
and the mantle appear with increasing distinctness ; so that towards 
the end of the second period the foetus already presents the charac- 
teristic form of the Cephalopoda. At this period, also, appear the 
fins, the third pair of arms, and the nervous and arterial centres. 
On the fifth day the constriction between the foetus and the vitel- 
line vesicle has become much deeper. The vitelline vesicle begins 
to perform alternate movements of contraction and expansion of its 
two lamellze. These movements are due to very delicate fibres, 
similar to those which M.Mecznikow has described in the amnios of 
the scorpion. Below the vitelline vesicle the two cylindrical parts 
which form the true lateral parts of the embryo (that is to say, the 
cephalic sinuses of M. Kdélliker) become prominent. Their outer 
region is divided into two parts, of which the upper contains the 
eyes and the ophthalmic ganglia, whilst the lower contains the 
cartilages and the lateral branches of the vitellus of nutrition. Fur- 
ther back the mantle and the parts of the body clothed by it are 
seen. The foetus terminates below in the two projecting fins. On 
the fifth day, when the number of arms is still only three pairs, 
none of these appendages is yet furnished with suckers. 
At the end of this second period the two halves of the siphon ap- 
proach and become united to form an unpaired organ ; the posterior 
part of the intestinal canal (independently of the anterior) divides 
into two cavities, the rectum and the ink-bag, the walls of which are 
formed at the expense of the outer lamella; the two pericardia 
appear at first as solid masses immediately below the branchie ; the 
eye becomes surrounded by a layer of pigment, in which, when 
