80 THOS. H. MONTGOMERY jr., 



2nd ovocyte 2 semivalent chromosomes which 



split into 4 semivalent „ 



ovum 2 „ „ 



In these cases, as in all others, we find that besides the alreadj-^ 

 recognized reduction in the mass and number of the chromosomes, the 

 chromosomes of the ovum and spermatid are semivalent on comparison 

 with those of the ovogonia and spermatogonia. But it would follow 

 from this, that in fécondation each chromosome, those derived from 

 the male as well as those from the female, are semivalent, and hence 

 that the valence of the chromosomes of the 1st cleavage spindle would 

 be only half of that of those of the spermatogonia or ovogonia. This 

 apparent discrepancy is bridged over by the consideration of the 

 phenomena during the growth period of the ovocytes and spermato- 

 cytes: during this period the amount of the chromatin is at least 

 doubled. If anyone thinks that this statement rests on theoretical 

 deduction alone, let him compare the volume of the chromosomes in 

 the spermatogonia of Pentatoma with the volume of them in the 1st 

 spermatocytes: in this object the amount of the chromatin seems to 

 be more than doubled. It follows then, that though the chromosomes 

 of the ovum and spermatid are semivalent on comparison with those 

 of the ovogonia and spermatogonia, they are nevertheless potentially 

 univalent, owing to the increase of the chromatin in the growth period. 

 It is of course generally assumed that the chromosomes of the 1st 

 cleavage spindle are univalent, but this conclusion has heretofore (by 

 most writers at least) been reached from a different line of reasoning. 



"Wistar Institute of Anatomy and Biology, 

 University of Pennsylvania, Philadelphia, U. S. A., 

 2nd April, 1898. 



