THE GERM-PLASM THEORY 299 
favor of this view are the facts that during mitosis 
the number and shape of the chromosomes are con- 
stant in every species (variations sometimes occur) 
and the complex series of processes in indirect nuclear 
division seems to be for the sole purpose of dividing 
the chromosomes equally between the daughter 
cells; even during the intervals (interkinesis) be- 
tween successive mitoses the chromosomes may be 
recognized in certain species as prochromosomes 
(see Digby, 1914, for review of literature). During 
the maturation of the germ cells chromosomes 
seem to play the most important réle, uniting in 
synapsis, and separating in the reducing division. 
The chromosomes of the minute, motile sperma- 
tozoa equal in number those of the comparatively 
enormous, passive egg; the spermatozodn consists 
almost entirely of chromatin, and this is the only 
substance present in the zygote that is equally 
contributed by both egg and spermatozodn. The 
processes following the penetration of the spermato- 
zoon into the egg bring about a combination of the 
chromosomes of the two gametes into a_ single 
nucleus; in certain animals at least some characters 
depend upon the presence of a certain chromosome, 
the X-chromosome; in certain cases of polyspermy 
the addition of extra male chromosomes seems to 
be the cause of the abnormal development of the egg. 
These and many other facts of chromosome be- 
havior that have been discovered by observations 
and experiments have convinced most biologists 
that the chromatin is the germ-plasm. 
