Fasten, Parthenogenetic Data. 
75 
such as those of Morgan, Shull, Whitney, and others have shown, 
is that the external conditions influence the methods of reproduc¬ 
tion, in fact determine whether the parthenogenetic or the sexual 
cycle shall make its appearance. 
The investigations described above all seem to pomelo the con¬ 
clusion that sex is determined through fertilization, the sperma- 
tozoan carrying the determining factor. 
This view has recently been substantiated by a considerable 
amount of careful investigation. McClung, Stevens, Morgan, 
Wilson, Payne and others have shown that in a great many Inver¬ 
tebrates, two kinds of spermatozoa are produced, differing from 
each other by one chromosome, or by a group of chromosomes, 
called the accessory, or the x-chromosome. All the eggs pro¬ 
duced, on the other hand, contain this x-element (except in a few 
cases). In some forms, it was also found that the spermatozoa 
each contained an accessory chromosome, but that in half the 
sperm this chromosome was larger in its chromatin content than 
in the other half. These were respectively called the x and the 
y-elements. 
It was then shown that a female was produced whenever an 
egg was fertilized by a sperm containing the x-chromosome, while 
a male resulted when an egg was fertilized by a sperm without 
the x-element, or by one containing the y-element. 
In the Vertebrates, Guyer was the first to show the existence 
of ao x-element, and other investigators have since brought forth 
similar evidence. 
As already stated, this postulation of sex determination can 
be extended to parthenogenetic forms. In species producing by 
this method, the fertilized eggs always develop through partheno¬ 
genesis, although in some cases, as in Daphnia, Aphis , and Hyda- 
tina, both males and females may develop from the unfertilized 
eggs. 
In the Aphids and Phylloxera, Morgan and Von Baer have 
shown that in the sexual phase, two types of sperm are produced, 
one-half containing the x-element, while the other half was with¬ 
out it. These investigators further showed that the portion of the 
spermatozoa without the accessory chromosome all die before 
coming to maturity, and on that account the male germ cells fer- 
