Reduction in Animals and Plants . 
435 
to them all, just in the same way that the formation and 
division of spore mother-cells marks a corresponding stage in 
the evolution of the reproductive elements in various forms 
of plants. It can, in fact, be used as a point from which to 
reckon the whole course of the spermatogenesis, just in the 
same way that we may speak in plants of the generations 
before and after the formation of the spore mother-cells. 
Now one of the most remarkable features of the spermatic 
heterotype in animals lies in the fact that the number of 
the chromosomes (i. e. separate chromatic elements) appearing 
in its prophase is always half that of the preceding homo- 
types. In like manner the number of the chromosomes in 
the division of the spore mother-cells of plants is always 
half that in the preceding cellular generations ; and this 
number, in animals as in plants, seems generally to be 
retained throughout any subsequent mitosis that may occur 
after the heterotype has been introduced. 
From all this it will be evident, that there are known to 
exist changes in the reproductive cells of many animals which 
are sufficient in themselves to fulfil any physiological require- 
ments arising from the facts that the number of the chromo- 
somes is specifically fixed, and that it is necessary to halve 
this number before fertilization can occur. 
It will further have been seen that this halving of the 
number of the chromosomes is not brought about by any 
division at all, but occurs in the resting condition of the cells, 
before the prophase of the great heterotype begins. Now 
these changes which lead up to the prophase of this division, in 
which the reduced number of the chromosomes first appears, 
are marked in all those forms with which I am acquainted, by 
a peculiarly contracted condition of the nuclear chromatin, 
which will at once be recognized by any botanist who has 
paid attention to the corresponding changes before the division 
of the spore mother-cells in plants. 
To this very singular appearance in the early prophasis of 
the heterotype in animals, I have applied the term Synapsis, 
from the Greek avoaxj/ 1?, and we may thus speak of any 
