154 CANNON: STUDIES IN PLANr HYBRIDS 
count of the early history of the germ-nuclei. In speaking of the 
latter, Wilson says (/. c. 273): ‘‘A large number of observers are 
now agreed that during the growth-period preceding the matura- 
tion division, in both sexes, the nucleus of the mother-cell (sper- 
matogonium, odgonium), both in plants and in animals, passes 
through some changes preparatory to reduction at a very early 
period. Thus, in the egg the primary chromatin-rods are often 
present in the very young ovarian eggs, and from their first ap- 
pearance are already split longitudinally. Hacker made the inter- 
esting discovery that in some of the copepods (Canthocamptus, 
Cyclops) these double rods could be traced back continuously to 
a double spireme-thread, following immediately upon the division 
of the last generation of oogonia, and that at xo period is a true 
reticulum formed in the germinal vesicle, In the following year 
Rickert made a precisely similar discovery in the case of sela- 
chians. After division of the last generation of odgonia the 
daughter-chromosomes do not give rise to a reticulum, but split 
lengthwise, and persist in this condition throughout the entire 
growth-period of the egg. Rickert therefore concluded that the 
germinal vesicle of the selachians is to be regarded as a ‘ daughter- 
spireme of the odgonium (Ur-cz) grown to enormous dimensions, 
the chromosomes of which are doubled and arranged in pairs.’ 
In this case their number seems to be at first the somatic number 
(thirty-six) which is afterward halved by the conjugation of the 
elements two and two (Rickert), as in Lumbricus (Calkins). It is, 
however, certain that in many cases (insects, copepods) the double 
rods first appear in the reduced number.”’ In another place (/. ¢. 
257), Wilson says: ‘‘A considerable number of observers have 
maintained that reduction may be effected by the union or conju- 
gation of chromosomes previously separate. This view agrees in 
principle with that of Rickert, Hacker and Vom Rath ; for bivalent 
chromosomes assumed by these authors may be conceived as two 
conjugated chromosomes.”’ It should be added further that Mont- 
gomery * and evidently Guyer as well + consider the conjugating 
chromosomes to be we balike Parentage. chica the hetero- 
oe wie 5 oh ahaa ett 
* eS, A Sade of the Siusiaiatias of he Gans Cells of the Metazoa, 
Trans. Am. Phil. Soc. 20: 1g01. 
¢ Guyer. Science. II. 15: 530. 1902. 
