140 THE GERM-PI^SM 



numerous enemies, but in this place I must refrain from includ- 

 ing such a possible occurrence in the argument. 



The following explanation of the phenomena, however, still 

 remains. The first division of the ovum separates the group of 

 determinants into two, viz., that for the right and that for the 

 left half of the body ; each of these groups does not constitute 

 a perfect germ-plasm, as each determinant it contains is not 

 doubled ; but it is very probable that the ids are capable under 

 certain circumstances of dividing in such a way that each be- 

 comes doubled. Such a germ-plasm could not contain in 

 potentia a birthmark, or any other asymmetrical peculiarity of 

 the other side of the body, but it would be able to give rise to 

 a complete animal. The destruction or mechanical removal of 

 one segmentation-cell in the first stage of segmentation may be 

 the primary cause of the doubling of the ids in the other cell. 



The capability of becoming doubled, which the undivided 

 germ-plasm possesses in certain cases, may be mentioned in 

 support of this view of the regeneration of an isolated cell in the 

 first stage of segmentation. The fact that in each integral 

 division of the cell and nucleus, a longitudinal splitting of the 

 nuclear rods and their contained macrosomes occurs, shows 

 that the ids are as a rule capable of growth and of doubling 

 their number by division. The assumption of a doubling of the 

 ids of germ-plasm must be made in dealing with the origin 

 of identical twins, i.e. those twins in which we must suppose 

 that the division of the nucleus of the ovum from which they 

 arise occurs after and not before fertilisation ; for otherwise the 

 embryos could not be identical, as two spermatozoa would then 

 take part in the process. In the case of facultative partheno- 

 genesis, a doubling probably also occurs in the ids and idants 

 of the ovum, half of them having previously been removed 

 by the 'reducing divisions.' 



The formation of an entire embryo by the regeneration of one 

 of the two first blastomeres admits, however, of another inter- 

 pretation. Ascidians multiply very freely by budding, and not 

 only by sexual reproduction. It is true that this is not the case 

 with sea-urchins, but the power of regeneration which these 

 animals possess is unusually great. This fact was explained in 

 the present chapter by assuming that certain idic stages of 

 ontogeny are provided with an ' accessory idioplasm,' consisting 

 of the determinants required for regeneration. In a subsequent 



