352 ORGANIC GROWTH sec. 



by the inheritance of the properties so acquired. Similarly, 

 only by modification of the original ordinary protoplasm, in 

 consequence of the constant exercise of nervous action, can 

 conducting nerve-fibres and nerve-cells have arisen. Spon- 

 taneous variations of the germ-plasm cannot possibly have 

 produced a system of organs so wonderfully delicate, both 

 morphologically and physiologically, exclusively as a means 

 of relation between the organism and the external world. 

 This must have arisen through external influences and 

 exercise. 



The fibrillar network above described occurs also in the 

 germinal vesicle of the egg-cell, even with the radial arrange- 

 ment round the germinal spot. That which may afterwards 

 become the path of nervous impulse is originally the path of 

 nourishment, and the radiation of the strands to and from a 

 central point is the most effective for the latter purpose. In 

 the streaming of the protoplasm of vegetable- cells, e.g. in the 

 hairs of Tradescantia, we see the commencement of this 

 typical arrangement of protoplasmic threads. The protoplasm 

 flows as though driven by a secret force — as also in the Fora- 

 minifera — towards the central point and then again from it : 

 the central point being the nucleus which originates and 

 governs the vital processes in the cell. Such paths, at first 

 fluid, became fixed, and finally, where they had to convey 

 nervous stimuli, became nervous fibrils. 



The same fibrillar network in the nuclei of the germinal 

 cells, in the germinal vesicle, is considered by Weismann, on 

 the other hand, as the idioplasm, i.e. the firm substance which 

 conveys the characters of the species from generation to 

 generation. 



I have already in the Medusce laid weight on the fact 

 that only in the ova of animals are the nuclei so extraordin- 

 arily large as in the ganglion-cells, and have attempted to 

 explain " the prominent part played by the nucleus in ova, 



