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POPULAR SCIENCE MONTHLY. 



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largely given to Weismann. The method of reduction imagined by him 

 accounted for the origin of variation in the offspring, and thus fur- 

 nished him with an explanation of the advantages, and therefore the 

 existence of sexual reproduction. It was the attempt to verify or 

 reject his theory which called forth so many monographs on this 

 subject. 



We shall not here go into 

 details of the process of the re- 

 duction of the number of chro- 

 mosomes. The most favorable 

 objects for demonstrating many 

 of the processes of maturation 

 and fertilization are the repro- 

 ductive cells of Ascaris. In 

 Ascaris each of the nuclei is 

 finally seen to contain two 

 chromatic threads and the ulti- 

 mate and intimate process of 

 fertilization is seen from the 

 photographs to be the union of 

 these two groups of two chromo- 

 somes into one group of four 

 chromosomes ready to divide. 

 The dynamic agents in their 

 union are two darkly stained 

 bodies from which rays eman- 

 ate, the centrosomes. In the 

 present case one of these enters 

 the egg with the sperm and this 

 is the usual method of its origin 

 in developing eggs. At the 

 completion of the process of fer- 

 tilization we have a cell con- 

 taining a nucleus with the num- 

 ber of chromosomes normal to 

 the species. One half of these 

 chromosomes came from the 

 father, one half from the 

 mother. When this cell divides 

 each one of the chromosomes 

 divides longitudinally, and one 

 half of each chromosome goes to one of the new cells and the other half 

 to the other cell. The observations of Rlickert, Hacker and Moenkhaus 



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Fig. 12. The Earliest Stages in the De. 



VELOPJIENT OF THE NEW INDIVIDUAL, a. The 



separation of halves of the chromatic threads. 

 b. Further separation and beginning of division 

 of the egg. c. Complete separation and complete 

 division of the egg. d. The two cells with their 

 chromatic threads ready to divide again. 



