500 Messrs. Fanner. Moore, and Walker. On (/ 6W/.s [Dee. 8, 



extending as it does to minute points of detail, can hardly be destitute 

 of grave significance. 



In order, however, to make the position clear, it will be necessary 

 briefly to consider the essential features in which the gametogenic* 

 tissues which are destined to become the reproductive elements are 

 found to differ from the other elements or cells of which the body or 

 soma of an animal or plant is elsewhere composed. 



When the egg of an animal or a plant segments to give rise to an 

 organism, the nuclei of all the resulting cells are found to contain a 

 definite number of chromosomes during each nuclear division. These 

 chromosomes pass through a constant series of evolutionary changes. 

 At first the material out of which they originate appears as an aggrega- 

 tion of granules of a stainable substance (chromatin) which finally 

 gives rise to definite structures, the chromosomes. These latter are 

 constant in number for each species of animal or plant, and each of 

 them divides longitudinally into two daughter-chromosomes. 



The chromosomes at this period of division become arranged in a 

 very definite manner on the spindle, frequently appearing as V's with 

 the apex directed towards the axis of the spindle. The daughter-nuclei 

 arc formed by the distribution to either pole of the respective halves of 

 each original chromosome, and the nuclei thus formed may then enter on 

 a condition of complete rest. Whenever new somatic cells are formed 

 in the body the nuclei pass through identically similar phases. But in 

 every individual there are certain gametogenic cells that are destined to 

 give rise, not to the specialised tissues, but to the sexual reproductive 

 elements. Such cells may be differentiated at a very early period in 

 the embryonic ontogeny of the organism, or they may only become 

 recognisable at a later stage. At whatever stage they may be formed, 

 however, their further history is entirely different from that of the 

 surrounding somatic tissues. The difference in question first becomes 

 apparent as the cell commences its preparation for division, and 

 it is distinguished both by its highly peculiar appearance, and by the 

 fact that atj a definite stage in the multiplication of the cells of this 

 reproductive tissue, each unit that is about to give rise to actual 

 reproductive cells passes through a series of metamorphoses wholly 

 dissimilar from those of the surrounding tissues as well as of those of 

 the antecedent cells by the division of which such a cell has sprung. 

 To the peculiar form of mitosis associated with this metamorphosis the 

 term Iwterotype has been applied, and it is a characteristic and inter- 

 polated stage in the reproductive cycle of all sexually propagating 

 higher animals and plants. 



* The term gametogenic^ as here proposed, is also intended to include the 

 primary sporogenous tissue of plants; in spite of possible objections that may bj 

 raised, wo have decided on this course to secure consistency of expression, \_3ol e 

 added December 16, 1903.] 



