FERTILISATION. 63 
It is important to understand that in ordinary mitosis or cell-division, 
each daughter-cell gets an absolutely similar half of each chromosome 
of the mother-cell, whereas in meiotic division the daughter-cells get 
dissimilar halves. 
’ A very important fact, discovered by Farmer, Moore, and Walker, is 
that the meiotic phase occurs among the cells of malignant growths 
(cancer). ‘*Through the action of one or several different causes at 
present unknown, certain cells of the soma, passing out of co-ordination, 
go through the meiotic phase and produce a number of generations of 
cells that live upon the parent organism in a parasitic manner.” 
Fertilisation.—In the seventeenth and eighteenth cen- 
turies, some naturalists, nicknamed “ ovists,” believed that 
the ovum was all-important, only needing the sperm’s 
awakening touch to begin unfolding the miniature model 
which it contained. Others, nicknamed “animalculists,” 
were equally confident that the sperm was essential, though 
it required to be fed by the ovum. Even after it was 
recognised that both kinds of reproductive elements were 
essential, many thought that their, actual contact was un- 
necessary, that fertilisation might be effected by an aura 
seminalis. Though spermatozoa were distinctly seen by 
Hamm and Leeuwenhoek in 1679, their actual union with 
ova was not observed till 1843, when Martin Barry detected 
it in the rabbit. i 
Of the many facts which we now know about fertilisation, 
the following are the most important :— 
(1) Apart from the occurrence of parthenogenesis in a 
few of the lower animals, an ovum begins to divide only 
after a spermatozoon has united with it. After one sper- 
matozoon has entered the ovum, the latter ceases to be 
receptive, and other spermatozoa are excluded. If, as 
rarely happens, several spermatozoa effect an entrance into 
the ovum, the result is usually some abnormality. It is 
said, however, that the entrance of numerous spermatozoa 
(polyspermy) is frequent in insects and Elasmobranch 
fishes. fans 
(2) The union of spermatozoon and ovum is very 
intimate ; the nucleu$ of the spermatozoon and the reduced 
nucleus of the ovum approach one another, combining to 
form a unified nucleus. ; 
(3) The ovum centrosome disappears before fertilisation, 
and it is a centrosome introduced by the spermatozoon that 
