XII.] CONJUGATION AND SEXUAL REPRODUCTION. II9 



of multiplication has lasted for a considerable time, the cells pass 

 into the second stage, — that of the ' mother-cells of sperma- 

 tozoa.' They cease to multiply, grow considerably, and their 

 nuclei pass into the resting condition, viz. the condition of a 

 nuclear network into which the loops break up. When these 

 cells have reached their full size they enter upon the preparation 

 for fresh divisions, which are only two in number and rapidly 

 follow each other. As soon as these are over, the whole 

 development is complete. It is this last stage which brings 

 about the 'reducing division ' which I had predicted. The finely 

 divided chromatin bodies contained in the nuclear network build 

 up eight long, thin rods or threads, which afterwards shorten 

 and form thicker rods, arranged by means of the pole-cor- 

 puscles or centrosomata, which act in such a manner that four 

 rods are turned toward one pole and four toward the other. A 

 division of the nucleus and of the cell now follows resulting in 

 the formation of two daughter-cells, each of which contains as 

 many nuclear loops as the original sperm-cells, i. e. four. This 

 division is followed immediately by another on the same plan, 

 but without any intervening resting stage : the number of 

 nuclear rods is therefore again halved, so that each daughter- 

 cell of the second order contains but two. 



Hence the number of nuclear rods is at first increased from 

 four to eight, and then by two consecutive divisions, this latter 

 number is first halved and then quartered, the final result being 

 a halving of the number of rods in the original sperm-cells. 



It is well known that precisely the same results are brought 

 about by those divisions of the ovum which give rise to the polar 

 bodies. In the egg the nuclear rods are first doubled and then, 

 by two consecutive divisions, reduced to half their original 

 number. In all essentials, the development of the ovum passes 

 through precisely the same process as that of the spermatozoa. 

 The first two stages, described by O. Hertwig, in the develop- 

 ment of the spermatozoa I also find in the formation of the &gg. 

 The primitive ova correspond to the primitive sperm-cells, the 

 mother-cell of the ova, or the mature full-sized egg, immediately 

 before reduction by division, corresponds to the mother-cell of 

 the spermatozoa, the only difference being that the egg in this, 

 the second stage, has, as a rule, attained its definite shape and 

 size and is surrounded by its membranes, and that the two last 



