946. ; The American Naturalist. [October, 
in the other cases the material was not suited to a decision on this 
point; the author thinks this differentiation between the nuclei of 
somatic and sexual cells may well be common to all the Ascaride. 
A second subject taken up by Oscar Meyer in this paper is the ori- 
gin of the centrosomes in the eggs of Strongylus tetracanthus. By the 
methods employed no centrosome could be found near the female pro- 
nucleus. The sperm-head is, on the other hand, accompanied by a 
very marked system of. radiations surrounding an evident centrosome. 
As the male pronucleus approaches the female pronucleus two systems 
of radiations and two centrosomes are formed by the division of the 
single centrosome that accompanied the male pronucleus. When the 
pronuclei are united these two centrosomes become the centrosomes of 
the first cleavage spindle. In some abnormal cases the female pronu- 
cleus has a centrosome close to it, but this probably migrates from the 
male pronucleus. It thus seems that i in this egg the centrosomes arise 
only in connection with the sperm. 
The third problem taken up by the author is the question as to the 
nature of the difference between the two kinds of Ascaris megalocephala. 
Boveri found that some individuals have two chromosomes in each egg 
or sperm while others have but one. The former have been called the 
variety bivalens, the latter univalens. 
Oscar Meyer examined 154 horses and found 19 infected with this 
parasite, 10 with the variety univalens, 8 with bivalens and 1 with both 
univalens and bivalens. 
A careful examination of the external and internal anatomy and 
histology of both kinds failed to reveal any difference except in the 
sexual products. The eggs of bivalens measure 78-88 and those of 
univalens only 65-70 microns. The sperms are larger in bivalens and 
have a nucleus twice as large as in wnivalens. 
The two kinds are very closely related and may, it seems, interbreed; 
at least the occurrence of eggs with three chromosomes as well as the 
finding of eggs of univalens penetrated by very large sperms points to 
such a conclusion. Copulation between the two kinds seems estab- 
lished by the discovery of worms with both sizes of sperms in the same 
egg-tube. A consideration of the numbers of apparent crosses so 
formed as compared with the possibilities that result from the presence 
of both kinds of sperm, leads to the conclusion that the crosses are not 
as frequent as they might be and that there may be some impediment 
to interbreeding. In other words the two kinds of Ascaris seem to be 
somewhat separated as physiological varieties in spite of their very 
close morphological relationship. 
