Biology 683 



system formed by chains of nerve-cells and their outgrowths. 

 The usual motor nerve-cell (i.e., a nerve-cell concerned in sending 

 messages to muscles and glands) has a short outgrowth within 

 the spinal cord, whose branches connect with the branches of 

 several other nerve-cells of different sorts, but the characteristic 

 feature is a very long nerve-fibre leading to the muscle con- 

 cerned. This, in the case of the nerve supplying the muscle of 

 the foot, may be several feet in length. 



The most extraordinary cells in the body are probably those 

 in the fore-brain, or main part of the fore-brain, in the layer 

 which is the seat of the process of thought. The figure (facing 

 page 684) will show the immense complexity of their interlacing 

 branches a complexity which presumably makes possible the 

 amazing process which we call the association of ideas. 



Finally, the act of reproduction itself is carried out by 

 special cells which are formed in the reproduction organs and 

 set free when mature. In early stages, both types of reproductive 

 cells have the same rounded form and large nucleus. But, while 

 the female cell or ovum remains rounded, and increases enor- 

 mously in size by the accumulation within itself of yolk-grains 

 as reserve food material, the male cells remain small and finally 

 become transformed into spermatozoa. The nucleus condenses 

 and elongates to form the so-called head, and the rest of the cell 

 is pulled out to form a long, actively vibrating tail, by means 

 of which the sperm propels itself and eventually, it may be, 

 reaches the ovum. 



4 

 The Body a Huge Cell-State 



Cells are for the most part quite microscopic in size. Human 

 red blood corpuscles, for instance, are so tiny that, in the volume 

 of blood equal to a cube one millimetre each way ( about TTTTTT 

 of a cubic inch) , some five millions of them are floating roughly 

 the population of London. As the amount of blood in an average 

 man amounts to about 7 Ibs. by weight, this figure must be 



