1904.] MONTGOMERY — CELLULAR IJASIS OF HEREDITY. 11 



as many chromosomes, and that their mass is the same, as those con- 

 tained in the egg. Further, it is proved that in normal fertiliza- 

 tion only one spermatozoon enters the egg, and that when more 

 than one enters the development is abnormal. The proof that 

 both egg chromosomes and sperm chromosomes have an approxi- 

 mately equal role in determining the growth of the embryo has 

 been shown by Boveri (/. c.) by crossing different species of sea- 

 urchins, and by analyzing the results of fertilizing an egg with two 

 or more spermatozoa. 



Now each act of fertilization would necessarily double the nor- 

 mal number of chromosomes, since the spermatozoon introduces as 

 many as are already present in the egg, were there not some pro- 

 cess to obviate this. There is such a process, and it is known as the 

 *' reduction in number of the chromosomes." The last two divis- 

 ions of the germ cells, preceding the act of fertilization and pre- 

 paring them for it, are known as the maturation divisions ; and it 

 has been known for some fifteen years past that in these divisions 

 each germ cell has only one-half the normal number of chromo- 

 somes. It is also proven that the ripe egg cell, as well as the ripe 

 spermatozoon, has only one-half the number of chromosomes char- 

 acteristic of the species. It is further known (since the work of 

 Henking and of O. Hertwig, in 1890) that the processes involved in 

 producing this result are essentially the same in both germ cells. 

 Accordingly, by this preliminary halving of the number in each 

 germ cell before fertilization, the germ cells on conjugation each 

 contribute only one-half the normal number, with the result that 

 the normal number is restored. But this preliminary reduction in 

 number has a broader meaning than this. 



Before the first maturation division of the germ cell is accom- 

 plished there takes place a pairing of the chromosomes, so that 

 instead of, e.g., four single (univalent) chromosomes there are two 

 double (bivalent) ones (Montgomery, Spermatogenesis of Peripa- 

 tus, 1899). These become so arranged that one of the two matu- 

 ration divisions results in separating chromosomes that are split 

 longitudinally, just as in any other cell division ; but the other 

 maturation division removes entire chromosomes from each other 

 by separating the tAvo chromosomes of each pair, and thereby 

 reduces the number of the chromosomes to one-half. That is 

 definitely known for certain species. 



