76 THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
characteristically present in the cells of malignant growths.* Again, it has 
been ascertained, as will be seen in the sequel, that during the maiotic change 
peculiarities occur in the position and behaviour of the centrosomes.t and 
these peculiarities have also been shown to characterise the development of 
cells during the formation of malignant growths. 
Thus in many ways it is desirable that a detailed account of the maiotic 
phenomena in mammals should be available, and the present memoir contains 
that part of such a history as is afforded by the study of the development of 
the male sexual cell. In a future publication it is intended that the develop- 
ment of the female sexual cells shall be similarly treated. But it may be 
pointed out that although the maiotic phenomena in the case of eggs gener- 
ally, is vastly more difficult to follow ; little, if any, new light will really 
thereby be thrown upon the subject. The broad features of the development 
of the egg in mammalia is sufficiently well-known to show already that its 
history will in all its cardinal features be parallel to the history of the male 
cells themselves. The only important matter to be readily obtained through 
an investigation of maiosis in female mammals will be in relation to the 
probable similarity, or identity, of certain of the so-called yolk nuclei with 
the archoplasmic vesicles, or Phmmer's bodies, in the male. This matter is 
at present obscure, on account of the fact that bodies which are unquestion- 
ably of diverse origin, are included under the term yolk nucleus, and have 
not yet been sorted out into different and specific categories. 
In dealing with a class of organisms so closely knit together as the 
mammals, it is not to be expected that there will be much diversity or 
important differences in the maiotic phenomena of different types, and so far 
as we know the whole course of events could be successfully illustrated 
through an examination of any single form. We have ourselves examined 
the maiotic process in men, baboons, bats, dogs, cats, rabbits, mice, guinea- 
pigs, hedgehogs, bulls, and the duck-bill platypus. On the whole, among 
these types, we have found that the guinea-pig is in many ways the best. 
Yet, as will be seen in the sequel, there are certain stages in the maiosis of 
this animal which are better illustrated by other types, consequently in what 
follows we have used the guinea-pig as a mammalian example perhaps best 
suited to illustrate the maiotic phenomena of the group; but have 
supplemented this example when necessary by observations based upon the 
examination of other forms. 
For the sake of convenience we have in this, as in our former conjoint 
work, % considered the development of the sexual cells, as naturally divided 
into three stages : — Pre-maiotic, maiotic, and post-maiotic. The pre-maiotic 
period includes all that series of cell divisions which extends from the 
first segmentation of the egg to the prophase of the first maiotic (heterotype) 
division. The maiotic period embraces the heterotype and homotype divi- 
sions, and the intervening rest ; while in mammals, as apparently in all 
animals, the post-maiotic period is only represented by the resting condition 
of the spermatid, or the egg after the second polar body has been extruded, 
there being in animals apparently no such post-maiotic divisions as generally 
* J. B. Farmer, J. E. S. Moore, and C. E. Walker. Pro. Roy. Soc, 1905. 
t See also J. E. S. Moore. Int. Monat. /., Anat. u. Phys., 1894. 
