105 M. Ussow’s Zoologico-Embryological Investigations. 
commence outside the body of the parent. The segmentation 
of the formative vitellus of the Cephalopoda greatly reminds 
us, as regards its form, of the segmentation of the eggs of 
birds * and Cheloniat. In all the four species of Cephalopoda 
investigated by me it is ¢rregular. ‘Lhe division of the pro- 
toplasm of the formative vitellus commences in its thickened 
central part, and spreads towards the attenuated peripheral part, 
which uniformly surrounds the whole surface of the nutritive 
vitellus. The latter takes no part in the segmentation pro- 
cess ( partial segmentation’’). One of the chief causes of 
the segmentation of the formative vitellus seems to be the 
great mobility of its protoplasm, and the changes of position 
of its heaviest parts, the darkest-coloured granules. ‘The 
segmentation always begins in the vicinity of the nuclei of 
the segmentation-cells (spheres of segmentation) or segments ; 
and the close of the complete cleavage (by longitudinal or 
afterwards transverse division) coincides with the complete 
separation of the nuclei. At first all the cleavages appear only 
at the surface of the formative vitellus, but then eradually 
penetrate by deepening to the lowest layers of the proto- 
plasm. 
The original or first furrow}, which divides the whole 
formative vitellus into two equal segments lying side by side, 
is soon (in about two hours) intersected at right angles by a 
second furrow. As the result of this division fowr equal seg- 
ments, enclosing four clear nuclei, are produced (the nucleoli 
are entirely deficient). In the central point there is produced 
a very inconsiderable clear interspace, which in the sequel 
soon disappears. ‘The subsequent cleavages of the formative 
vitellus are irregular; from four segments there are formed 
(in four hours), first sew, and then eaght equal segments. In 
the period between the formation of the six and of the eight 
segments, there are produced at the centre of union of the 
furrows, in the earliest moments of the appearance of the two 
narrowest segments, by constriction of the apices of these, two 
primitive cells or spheres of segmentation (approximately be- 
tween the third and fourth hour of the process of segmenta- 
tion). Irom the two of the eight segments which are situated 
* Coste, Hist. part. et gén. des corps organisés, p. 287, pl. ii. 
+ Agassiz, Contrib. to the Nat. Hist. of the United States, ii. 
if In Lolig go, Seprola, and Argonauta this furrow appears directly beneath 
the micropyle, in the centre of the formative vitellus; in Sepa sometimes 
a little to one side, which I regard as an abnormal phenomenon, as also 
that I once in Sepiola found the. segmentation on the lower obtuse pole of 
the ovum. The hours mentioned in the following description of the pro- 
cess of segmentation relate to Seprola and Loligo. 
