1K3 NATURAL SCIENCE [August 



Three modes of nuclear reduction have been described : — 



( Jagg J, — l n Elasmobranchs, some Amphibia, and Mammals ; in 

 Flowering Plants, some Archegoniates, and Fucaceae. The nuclear 

 network resolves itself at once into half the previous number of 

 segments; certain modifications occur which do not alter the 

 principle of the matter. Here there is clearly no true 'reduction,' 

 any more than if a man received ten shillings from his father and 

 left five florins to each of his two sons. 



Case II. — In Sagitta and Ascaris, in some Gasteropods, and 

 in Liverworts, a modification of this process has been made out, 

 which was first noted by Boveri and Hertwig in 1890; while 

 its relation to Case I. was dwelt on by me in 1891. Here the 

 reduced number of segments appears in a certain cell ; but after 

 the first splitting for the coming division a second splitting of each 

 occurs, so that at the first division of the nucleus each daughter 

 nucleus receives the reduced number of segments ready split for the 

 second division ; and at the second division the again new daughter- 

 nuclei receive each its own set. This is merely a displacement in 

 time of the two successive splittings for two successive divisions of 

 the nucleus. Similarly, often when a cell is going to undergo two 

 or more successive divisions, the nucleus underooes the consecutive 

 divisions before the cytoplasm divides once. The reduction is 

 essentially of the same character as in Case I. 



Case III. — The nucleus about to divide reveals a number of 

 tetrads or groups each of four segments; the number of groups is 

 only half the previous number of segments, and consequently the 

 total number of segments is twice the original number : the nucleus 

 divides twice, and at each division the several groups are halved 

 between the resulting nuclei ; hence it is clear that at the second 

 division each nucleus contains one segment from each group — i.e., 

 a number equal to the number of groups, and half the original 

 number of segments in previous cells of the cycle. 



Concerning the details of Case III. there are many conflicting 

 observations, many inconsistent explanations, many vain hypo- 

 theses based on the assumption that this is the typical mode of 

 nuclear reduction to which all others are to be forced to conform 

 by some Procrustean process. But we shall not go into these ; the 

 explanations that cover Cases I. and II. will cover Case III. also, 

 even if in the last there be something additional left over ; but the 

 explanation of this something can well wait until the facts them- 

 selves are better made out. 



Where does nuclear reduction occur ? In Metazoa, usually at 

 the first of the two cell-divisions that give rise to a brood of four 

 spermatozoa, or to the oosphere and the three polar bodies (abortive 

 oospheres) respectively : that is to say, at the inception of the forma- 



