WEISMANN'S THEORY OF THE GERMPLASM 63 



addition to its specific part of the hereditary mass, 

 the part that controls its normal course of develop- 

 ment, possesses an accessory idioplasm, an undivided 

 fragment of the germplasm, left behind to be ready 

 for unforeseen emergencies ; this part takes com- 

 mand when, in consequence of violence, a separated 

 part develops into the whole. 



But such an assumption does not go far enough, 

 if it be confined to the first cleavage-cells. By 

 compression of the frog's egg, I have shown that 

 the pole passing through the blastopore, which 

 coincides with the chief axis of the future embryo, 

 may assume different relations to the first segmenta- 

 tion-plane, sometimes coinciding with that, some- 

 times making a right or an acute angle with it. It 

 is clear that in each of these cases the embryonal- 

 cells take a different share in the formation of the 

 regions of the body, and that they must be fore- 

 endowed with the capacity of playing different 

 parts. 



The developmental history of double monsters 

 enforces the same doctrine ; such are common 

 among the embryos of fish, and rather less common 

 among chicks. From causes of which we are 

 ignorant two, instead of one, gastrula stages may 

 arise at separate regions of the germinal layer of 

 the egg. According to the position of these two 

 invaginations, which may be regarded as crystal- 

 lisation-points for the formation of the future 

 embryo, the cells of the germinal disc will be drawn 

 into the process of development, and, falling into 

 groups, will build up organs. In relation to this 



