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widely different group Mammalia. Although the types selected 
differ to a certain extent in detail inter se, they are yet sufficiently 
alike to allow a general statement of the minute structural changes 
which take place in the sperm -forming cells, after they have become 
distinguished from the indifferent elements in which they originate. 
With respect to the exact manner of this primary differentiation con- 
siderable controversy exists, and it appears to be by no means constant 
even among the Mammals themselves. But this inconstancy is not of 
much importance for my present purpose, as even when the multipli- 
cation of the regenerative cells is obviously mitotic, the figures are 
so small and confused, that it is impossible to count the chromosomes 
present during the division. I shall therefore term this time of re- 
generative multiplication the first period, because it is thus possible 
to divide the whole spermatogenesis into three successive stages, sepa- 
rated by two mitoses in which the numerical relations of the chromo- 
somes can be determined with more or less exactitude. 
Thus there is in the Rat 1) a period of indifferent cells terminated 
by a mitosis with apparently 16 centrosomes, both in the primary and 
daughter cells; 2) a period of growth during which the 16 elements 
are converted into 8, and terminated by a division in which the 
daughter nuclei (spermatids) still retain the number 8; 3) a period 
during which the spermatids are converted into spermatozoa. 
All these generations of cells contain the usual complement of 
Nebenkern and centrosomes, and it is only just prior to the second 
division, in which the spermatocytes are converted into spermatids, that 
accessory bodies make their appearance in the shape of a small in- 
tensely chromatic extra-nuclear particle (fig. 1 c), ultimately incorporated 
with one or other of the chromosomes, in the immediately succeeding 
division. 
In order to truly apprehend the real simplicity which underlies 
the apparent complication of mammalian spermatogenesis, it is necessary 
to follow in detail the evolution of these accessory bodies, from origin 
to fate in relation to the mature spermatozoa. 
Immediately prior to division, there appears at the side of the 
nucleus another structure, which I shall call the Lesser Nebenkern 
(fig. 1%), so that spermatocyte immediately before division contains: — 
1) A nucleus with 8 chromosomes. 2) An extra-nuclear chromatic body 
(fig. 1c) (Hermann’s chromatic body). 3) A body which I have termed 
the Lesser Nebenkern (fig. 1%). 4) The Nebenkern itself (Archoplasm) 
(fig. 1a). 5) Two centrosomes (fig. 15), not contained within the mass 
