1905.] NATURAL SCIENCES OF PHILADELPHIA. 189 



tions of spermatogonia, each with the normal number of univalent 

 chromosomes (the heterochromosomes will not be considered in this 

 place) ; all of their mitoses are equational. The last generation of them 

 produces the spermatocytes of the first order. At an early period in 

 these there takes place a pairing of the univalent chromosomes to form 

 bivalent ones, which may be a junction end to end or side to side.^ 

 This is in each case a pairing of a paternal chromosome (one derived 

 from the spermatid) with a maternal one (one from the ovotid). At 

 an early stage of the growth period the bivalent chromosomes become 

 more or less densely grouped, the synapsis stage, but the pairing of 

 the chromosomes may commence shortly before this time. After this 

 conjugation each univalent chromosome becomes longitudinally split, 

 and no second splitting follows the first. There may or may not be a 

 rest stage during the growth period, and when it occurs it may come 

 before or after the synapsis stage. In the first maturation mitosis 

 each bivalent chromosome undergoes a division in such a way that one 

 whole univalent element passes into one daughter cell, the other one 

 into the other cell ; this is a true reduction division in the sense of Weis- 

 mann, and accomplishes the reduction in number of the chromosomes; 

 their conjugation in the rest stage had not effected reduction, but only 

 the formation of pairs. The second maturation division is equational, 

 along the line of the longitudinal split, so that the spermatid receives 

 half the normal number, and each of them on comparison with those 

 of the first spermatocytes is semivalent, but on account of their increase 

 in size during the growth period virtually univalent. All the facts 

 speak for a strict preservation through the whole germinal cycle of the 

 individuality of the chromosomes. 



From the correspondence determined by Henking (1890) and Hert- 

 wig (1890) between spermatogenesis and ovogenesis, by the one for 

 insects and by the other for Ascaris, we might conclude that in all cases 

 of ovogenesis also prereduction occurs, as indeed has been described 

 for some animals. I think there is no sufficient evidence at present for 

 doubting this conclusion, and much in favor of it. Yet it must be 

 acknowledged that the ovogenetic processes are less easily analyzed, 



"^ Marechal is incorrect in stating that von Winiwarter (1900) first described 

 this process ; he simply reasoned that of three possible explanations of the origin 

 of the bivalent chromosomes this was the most probable. Henking (1890), 

 before von Winiwarter, had more conclusively argued for this, but did not see the 

 first steps in the process. I was the first (1900) to describe all the steps in this 

 series, and (1901a) to prove that each bivalent chromosome is formed by the 

 conjugation of a paternal and a maternal one — this corroborated in the next year 

 by Sutton (1902). Gross ('1904) is of course in error when he calls this the 

 "Hacker'sche Theorie." This important phenomenon is at last receiving rapid 

 confirmation from many sides. 



