378 
REPRODUCTION — ONTOGENY. 
Fig. 70.5.— Fgg of Limtura stagnnlh 
(L.), showing iis first segmentation and 
the expulsion of the polar globules, 
highly magnified (after Ray l.ankester). 
tiv.st proce.sses of cell division, the embryo devouring the rest of the ova 
in the capsule, which thus supply it with nutriment. 
The Ontogeny (orro, being; yh'os, race) or development of the in- 
tlividual from the ovum, known also as Embryology, during which the 
embryo passes through a series of larval stages or phases of development 
before acquiring adult form, which are a condensed and modified but 
evanescent repetition or recapitulation by the individual mollusk of 
the various forms and characters possessed by the whole chain of its 
ancestors, and is, therefore, an epitome of the Phylogeny or develop- 
ment of the race. 
Upon the maturation of the egg, and before its fertilization, it 
separates from the ovarian wall, the clear ])rotoplasm accumulating 
at the animal or formative pole, and 
the darker food yolk forming the 
opposite and larger segment distin- 
guished as the vegetative or nutritive 
})ole. The nucleus approaches the 
walls of the vitellus at the formative 
pole and divides mitotically, one half 
the nucleus being exi)elled as the first polar globule or directive 
corpuscle, the remaining moiety of the nucleus again undergoing 
division, and half the remaining nuclear matter is extruded as the 
second polar globule, thus leaving the germinal vesicle with only one- 
fimrth of the nuclear matter originally present, which is then known 
as the female pronucleus. 
Ui)on fertilization the head of the 
■Spermatozoon fu.ses longitudinally with 
the female pronucleus, and transverse 
segmentation or cell division of the 
combined nucleus takes place at the 
limit of its growth, the direction of 
the cleavage furrows being eiiuatorial or meridional, according as 
they coincide with or are perpendicular 
to the chief axis. 
The segmentation of the nucleus may 
he direct or indirect. 
Amitotis (d, without ; piros, thread) 
or direct division only takes place as a 
rough and ready means of division of the larger cells and rarely occurs. 
Fig. 706.— Early embryo of PisidiKvi 
pnsilluin (Gmelin), highly magnified, 
showing the meridional and equatorial 
cleavage grooves (after Ray Lankester). 
Zz 
> f ' 
Fig. 707. — Direct or Amitotic cell 
division, showing the nucleus in pro- 
cess of separating to form two 
daughter nuclei (after Carnoy). 
