80 THE NATURE AND ORIGIN OF LIVING MATTER. 



combinations. By these chemical changes a continuous formation 

 of energy takes place, which energy is given off as heat, or 

 sometimes also partly as mechanical motion, or in other ways. 

 Living matter is continually in a state of unstable chemical 

 equilibrium. 



By a preponderance of assimilation over waste the living 

 cell grows in size. A consideration of the statements just 

 enunciated will convince you how fundamentally different such 

 growth is from that, for example, of a crystal. The latter growth 

 is wonderful to contemplate, but it is a growth by accretion ; 

 each increment once formed is stable. The growth of the cell 

 usually ends in division, which, in the case of the amoeba, leads 

 to the formation of two individuals each resembling the parent 

 cell. But in the higher animals, the process of cell division 

 leads to more complex developments. A brief glance at these is 

 necessary for our purpose. 



The human ovum, not very different in structure from an 

 amoeba in the encysted stage, consists of a nucleated cell about 

 fifteen of a millimetere in diameter, forming a speck just visible 

 to the naked eye. The first stages of development consist, as in 

 much humbler forms of life, in the division of this cell into two, 

 four, sixteen, and more cells, forming a cluster, somewhat 

 resembling the form of a mulberry. As the cells multiply fluid 

 accumulates between them, and they form a minute vesicle, 

 round which the cells are grouped at first in two, then in three 

 layers. From these three layers of cells are developed by 

 successive steps all the marvellous complexity of the adult human 

 frame. The process by which this change occurs has to a great 

 extent been observed and mapped out. It is a wonderful history, 

 and the process by which each cell assumes its right place, and 

 each group of cells differentiates itself into the right tissue in 

 exactly the right situation, is entirely baffling to the imagination. 

 Let me very briefly glance at the developmental history of one 

 portion of the human frame. It is at first surprising to learn 

 that the whole nervous system is developed from ancestral cells, 

 which formed part of the external surface, or skin, of the 

 embryo. As the development of the individual is but a 

 recapitulation, with some modifications, of the development of 

 the race, this fact seems to take us back into a very remote past, 

 when the cells specially devoted to sense-perception, which would 

 naturally be situated near the surface, were not yet difl'eren- 



