The determination of sex in animal development. 707 



512 etc. The products of the primitive germ-cell are the primary 

 germ - cells , on one of which falls the lot of devel- 

 oping into an embryo. The remaining primary germ-cells 

 become the "sexual products" of this said embryo. They may be 

 obliged, as in the skate, to wander into the embryo, or, owing to the 

 mode of growth and evolution of the embryo, they may become en- 

 closed by it. 



The primary germ-cells are destined for future generations. After 

 a long resting phase, during which the embryo gradually manifests 

 itself, they begin to divide, and to form secondary germ-cells. It is 

 with the division of the primary germ-cells into secondary ones, that, 

 as revealed in the later pages of the present writing, the beginning 

 of the determination of sex for the following generation is bound up. 

 After a certain, usually limited, number of divisions these germ-cells 

 become oocytes or spermatocytes, in which the final step in the de- 

 termination of sex is taken by the numerical reduction of chromosomes. 

 When this has been effected, there remains nothing more to complete 

 the cycle of the germ-cells than that they should form gametes, and 

 this they usually do by two divisions. 



The greater portion of the present writing was completed — 

 practically as now published — six months ago. In the interval it 

 has undergone repeated examination and, where needed, revision. As 

 it began to take form from the moment, when the existence of two 

 sorts of eggs in the skate was ascertained , and as at that time 

 the writer was far from suspecting the general, if not universal, 

 occurrence of the forerunners of two forms of gametes in the male, 

 or of two categories of gametes themselves, the manuscript has acquired 

 a character, very different from that, which its present inditement 

 would give it. Its gradual growth may account for, and, I hope, ex- 

 cuse, some apparent, though not real , contradictions between its 

 earlier pages and the later ones. The question of re-writing it has 

 often been considered, albeit rejected from anxiety, lest such a course 

 should destroy the continuity of the argument. Many things, which 

 now appear so obvious to the writer, might be mentioned without 

 any evidence, were the manuscript to be re-cast. 



The form, in which the work^) leaves the hands of its author, 

 will at least enable the reader to follow the line of thought in the 



1) And read, as now published, before the Royal Society, Edinburgh, 

 on July 1st, 1901. 



