376 On the Reciprocal Relation of the Vital ^ Physical Forces. 



ment of motion in separate regions or portions of the whole tis- 

 sue, and ultimately in the entire body of the embryo when 

 formed. In this way the first movements of organs in the inte- 

 rior of the embryo are commenced. Thus motion is generated 

 in individual cells during the earlier stages of formation through 

 heat force derived from without, and becomes a fixed and inherent 

 power in one structure, while the same force may be evolved as 

 light in another. In the union of the cells, as centres of force 

 to form muscular tissue, a power of contractility is produced. 

 Thus muscular force is derived from heat, chemical change, and 

 gravity. These principles of formation of structure apply to 

 every period of change in the animal. I have mentioned in a paper 

 on the Circulatory Organs *, that " two modes of development 

 are in operation in the same animal : first that of growth or sim- 

 ple extension or enlargement of each individual part ; next that 

 of aggregation of two or more parts to form particular divisions 

 or regions of the body ;" and 1 have elsewhere f carried out 

 these views more in detail. Now, what is true with regard to the 

 compound parts of the already-formed embryo is equally true 

 with reference to the simple constituent structures through which 

 it is originated, and to the changes in one species as in another. 

 In the egg of the glow- worm, Lamjjyris, the cells in a portion of 

 the foundation layer, instead of uniting to form muscular or ner- 

 vous tissue, retain to a great extent their primary individuality, 

 and assuming somewhat of the condition of secretory organs, 

 evolve their vital force as light. Thus through heat supplied to 

 the ovum from without, light becomes a product in the organi- 

 zation of the embryo. I have seen light emitted from the lu- 

 minous organs at the moment the embryo is escaping from the 

 egg-shell. It may perhaps be urged that this light is simply that 

 of combustion, or is assimilated rather to the purely vegetative 

 processes of organization, than is due to, or is a form of vital 

 force. But this objection seems to be answered by the fact that 

 the light of the glow-worm is excited to greater vividness not 

 only by higher temperature of the surrounding medium, and an 

 acceleration of the respiratory and circulatory processes, as well 

 as by immersion in oxygen, but also by mechanical irritation of 

 the animal, and consequent excitement of nervous force. In this 

 respect, then, the production of light by the glow-worm seems 

 to bear analogy with the evolution of electricity through me- 

 chanical or other modes of exciting nervous force in the electrical 

 fishes ; and if this be the fact, we then have force derived from 

 without, in the form of heat, converted through organization into 

 vital force, and evolved as muscular contractility, light, electricity 

 or nervous power. 



* Phil. Trans. 1843, part 2. p. 244. f Linn. Trans, vol. xix. part 3. 



