610 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The question of the origin of the gametes has now to be considered. 
This has been most studied as to the origin of the female gamete, the ovum. 
The opinion currently held is that it originates from a germ epithelial cell. 
One admits, of course, that this view has done good service as a stop-gap, but 
there is really no fact in its favour. It makes a gamete originate from a 
somatic cell whose fellows, not so transformed, become ordinary protective 
epithelium. No satisfactory intermediate stages have been noted, and in 
view of the great fact that each gamete contains a certain proportion of 
heredity determinants, it is at present impossible to see how a gamete, the 
most highly organised cell in the body as to function, could arise from a 
germ epithelial cell. This view of its origin, further, gives no clue to the 
understanding of how a gamete becomes so specially endowed. 
A more promising view of the origin of the gametes is that they are 
ultimately derived from the primitive germ-cell mass from which the 
primitive germ cells are first formed. These primitive germ cells become 
reduced to gametes — spermatogenesis and oogenesis. The P.G.C. mass is set 
aside from the zygote at its very earliest period. The zygote is thus divided 
into a somatic portion containing one id, and the P.G.C. mass, necessarily 
containing thousands of ids. The former is thus the somatic portion of the 
zygote, giving rise to the individual, while the latter is the propagative 
part, the stirp in Galton’s phrase. The holophyte is thus merely the trustee 
of the propagative part. In the stages of zygotic development subsequent 
to the formation of the P.G.C. mass, the P.G.C.’s travel back into the 
somatic part via the body-stalk (yolk-stalk) and blastoderm before the 
embryo is differentiated as such, and mobilise on the Wolffian ridges, thus 
ultimately forming in each case the sexual gland, ovary or testis. 
Much detail on this point has yet to be worked out. Most important 
deductions can be made from this view : 
1. The P.G.C. mass is really an unreduced part of the zygote, and is 
thus zygotic. 
2. Each P.G.C. is potentially a zygote. 
3. The gamete is a reduced P.G.C., and this reduction normally prevents 
zygotic development of the P.G.C. 
4. To regain the power of zygotic development, gametes of separate 
adults must unite, the male gametes with the female gametes ; and thus 
fertilisation is gametic variation. 
5. The gametes contain certain of the heredity determinants, because 
they are derived directly and by reduction from an unreduced part of the 
zygote, the P.G.C. mass. 
6. The male zygote in all likelihood also gives off a primitive sperm-cell 
