THE FORMATION OF GF.RM-CELLS I97 



nected with certain of those of the mesoderm during several 

 generations of cells, and in each subsequent division certain of 

 the latter pass out alone into one of the daughter-cells, the 

 other retaining the primary constituents of the primarv germ- 

 cells in addition to those of the mesoderm. Finally, in the 

 ninth series of cells in the diagram — in which the processes are- 

 represented as greatly abbreviated — the separation of these two 

 sets of primary constituents occurs, and the first primarv germ- 

 cell (///' A':;-) is formed. 



So much is certain, and does not depend on any hypothesis. 

 Opinions may differ as to whether the cells situated in the germ- 

 ti-ack are to be described as real somatic cells. I have called 

 them so, because in many cases the germ-tracks extend far 

 beyond the period of embryogeny into the fully-developed func- 

 tional tissues, and because it can be proved that even cells which 

 are histologically differentiated 7nay produce germ-cells under 

 certain circiwistances . This occurs amongst plants — in the 

 prothallus of ferns, for instance — and also in the cells of certain 

 Polyzoa from which gemmation may take place, and which must 

 therefore contain inactive germ-plasm. In these cases it is 

 certain that real somatic cells are situated alone the germ- 

 tracks ; in all cases the cells of the germ-track are not 

 germ-cells from the first, and they always take part in the con- 

 struction of the body. And if we further consider that a large 

 number of somatic cells must contain accessory idioplasm of 

 some kind. — either that which will serve for simple regeneration, 

 or for the regeneration of more complex parts, or again, for the 

 formation of buds, — we can hardly assume that the character of 

 a somatic cell is thereby abolished : I can see no advantage in 

 objecting to describe a cell of t lie germ-track as a somatic cell. 



The change which the idioplasm of the cells constituting the 

 germ-track undergoes, can obviously only consist in its actii'e 

 portion gradually becoming separated off in the course of the 

 ontogenetic cell-divisions, so that ultimately the cell contains 

 germ-plasm only, which then stamps it as a germ-cell. Even 

 then the germ-plasm remains unalterable as long as this first 

 or primary germ-cell continues to produce others similar to 

 itself. The cells only become differentiated into spermatozoa 

 and ova when this multiplication ceases, and this presupposes 

 the splitting off of special spermatogenetic or ontogenetic de- 

 terminants. The disintegration of the germ-plasm which results 



