BEPRODUCTION AND HEREDITY 293 



continually formed, the so-called sperrnatogoriies (or spermato- 

 blasts). Adjoining the germ-zone is the zone of growth. In 

 this the spermatogonies develop and become spermatocytes of 

 the first order. These are, according to Yves Delage, the grand- 

 fathers of the spermatozoa proper, for from each spermatocyte 

 of the first order originate, by two successive divisions of four 

 cells each, the spermatids, which change into the mature sper- 

 matozoa. In the course of this transformation takes place the 

 reduction of the chromatosomes to half the normal number. 



I shall describe only the most frequent case, omitting all 

 deviations from this typical process, and choose for our illustra- 

 tion Ascaris megalocephala (the thread- worm of the horse) 

 which, as a result of the experiments of 0. Hertwig and 

 A. Brauer, is one of the best-known organisms. Like the body- 

 cells of this nematode, so its spermatocytes of the first order 

 contain as yet each four chromosomes. Before they proceed 

 to division, a doubling of the chromosomes takes place and 

 each two of these double-chromosomes approach each other, as 

 if impelled by a mysterious force. In this manner are formed 

 two groups of four, each of which consists of two double-chromo- 

 somes. This mutual seeking and uniting of two chromosomes 

 which were separated during the entire previous life of the 

 organism recalls, as Heider justly says, the process of conju- 

 gation in unicellulars. Now the whole complicated dividing 

 apparatus of the cell comes again into activity, the centro- 

 proceeds the characteristic radiating figure), move to opposite 

 some divides, and the two resulting parts (from each of which 

 nucleus-poles. Now the nucleus-membrane disappears, and 

 before our eyes is formed the ' equatorial plane.' A longitudinal 

 fission of the chromosomes need no longer take place, as the 

 chromosomes have already been doubled. 



The remaining course is normal. Two cells are formed, the 

 two spermatocytes of the second order, of which each has again 

 received four, that is, the normal number of chromosomes. But 

 while after each mitosis the nucleus is reconstructed and 

 returns to the rest stage, here the first cell-division is imme- 

 diately followed by a second. Again the four chromosomes 

 arrange themselves in each of the two spermatocytes of the 

 second order into a group of four, the centrosomes divide and 

 wander to the poles, and again we see the equatorial plane. But 



