192 



THE GERM-PLASM 



tacean (Cyclops), have indeed also proved that the behaviour of 

 the somatic segmentation cells is different from that ot the 

 primary sexual cell, but the process differs essentially from that 

 which occurs in Ascaris. When we are in possession of similar 

 observations on various types of animals, we shall be able to 

 recognise the essential parts of the process, and shall then be 

 in a position to offer an explanation of them. 



From a theoretical point of view, we must expect that the ids 

 of germ-plasm become doubled in the nucleus of the fertilised 

 egg-cell or even previously, one half of such a double id being 

 in the active condition in which it can undergo disintegration, 

 and the other being in the inactive and unalterable condition. 

 The former direct ontogeny, and the latter are passed on in a 

 passive condition to the primary sexual cells. As these, how- 

 ever, behave at first like somatic cells, — that is to say, they 

 multiply in a regular manner, and are distributed amongst defi- 

 nite series of cells to definite parts of the body, — they must 

 possess active idioplasm in addition to unalterable germ-plasm. 

 They must therefore contain more ids in their nuclear matter 

 than do the somatic cells. The above-mentioned observations 

 on Ascaris can thus be explained in accordance with our theory 

 up to this point, but more than this cannot be stated at present. 



2. The Germ-tracks 



Taking sexual reproduction only into consideration for the 

 present, the course of the germ-tracks in existing Metazoa ap- 

 parently varies both as regards its length and the direction it 

 takes. The germ-track is shortest in the Diptera, in which the 

 primary germ-cell becomes separated off in the first division of 

 the ovum, so that in this case we might speak of a division of the 

 ovum into a primary germ-cell and a primary somatic-cell. In 

 the DaphnidcB the germ-track is longer ; for, counting from the 

 ovum, five successive divisions occur before the primary germ- 

 cell is formed. In the free-swimming marine worm Sagifta it is 

 longer still, two primary germ-cells only appearing after ten or 

 more successive divisions have occurred, and tlie mass of 

 embryonic cells has already given rise to a gastrula-l.uva. In 

 other worms, such as the Nematodes, the primary germ-cells 

 become separated from the somatic cells in a still later genera- 

 tion of cells, which has so far not been actually determined: and 



