SEGMENTATION. 65 
of Magnesium chloride, and by other means). It seems, 
therefore, justifiable and useful to distinguish in ordinary 
fertilisation, (a) the mingling of the hereditary qualities of 
the two parents, and (4) an exciting or liberating stimulus 
which induces the ovum to divide. It should be noted 
that the chromosomes of the spermatozoon do not fuse with 
the chromosomes of the ovum when fertilisation occurs. 
There is some evidence for the view that they remain 
distinct from one another until maturation again takes 
place, and one theory of the reduction in the number of 
chromosomes which takes place at maturation, is that it 
involves the fusion in pairs of the paternal chromosomes 
with the maternal. 
In some insects there is an accessory chromosome 
present in one half of the spermatozoa. It has been 
interpreted as an element whose presence or absence 
determines whether the.offspring is to be male or female. 
Segmentation.—The different modes of division exhibited 
by fertilised egg-cells depend in great measure on the 
quantity and disposition of the passive and nutritive yolk 
material, which is often called deutoplasm, in contrast to 
the active and formative protoplasm. The pole of the ovum 
at which the formative protoplasm lies, and at which the 
spermatozoon enters, is often called the animal pole; the 
other, towards which the heavier yolk tends to sink, is called 
the vegetative pole. In the floating ova of some fish, how- 
ever, the yolk is uppermost, and the embryonic area 
lowest. 
In contrasting the chief modes of segmentation, it 
should be recognised that they are all connected by 
gradations. 
A. COMPLETE Divis1on—Holoblastic Segmentation 
(1) Eggs with little and diffuse yolk material divide completely into 
approximately equal cells, 
[or, Ova which are alecithal (z.e. without yolk) undergo approxi- 
mately equal holoblastic segmentation]. 
This is illustrated in most Sponges, most Ccelentera (Figs. 
31 (1) and 32), some ‘‘ Worms,” most Echinoderms, some 
Molluscs, all Tunicates, Amphioxus, and most Mammals. 
{2) Eggs with considerable yolk material accumulated towards one 
pole, divide completely, but into unequal cells, 
5 
