THE GROWTH OF THE BRAIN. 



tute the vertebrate group, to which man himself belongs, 

 and since the changes in the vertebrates arc those of 

 greatest interest, the facts presented will especially apply 

 to them. 



Minute anatomy has taught us that all animals are 

 composed of small structural elements cells. Those 

 microscopic portions of living matter, usually but a 

 small fraction of a millimeter in diameter, possess, 

 whatever their shape, two principal parts : a small and 

 denser portion centrally located, the nucleus, and sur- 

 rounding this a more fluid and bulkier portion, which 

 forms the body of the cell, the cytoplasm ; the latter 

 being so altered at its surface that its 

 outermost layer constitutes an enclosing 

 envelope, the cell-membrane. The cyto- 

 plasm and the enclosed nucleus are 

 only the most evident and not the sole 

 FIG. i Unferti- cons tituents in the typical cell, but the 

 lised human finer anatomy of these elements need 

 ovum, x 170 t be here described, 

 diam. (Nagel). 



C. Cytoplasm ; These are the units by the produc- 



N. Nucleus ;Z.p. {-Jon of \vhich the growing animal is 



formed, and of which the mature animal 



is principally composed, and therefore all of them 



are the lineal descendants of the ovum, which is, in 



its first form, but a single cell. The formation of 



the animal-body from this simplest beginning, with all 



that this implies of cell-multiplication, enlargement, and 



specialisation, is the result of changes first showing 



themselves within the ovum. (Fig. i.) 



In order that these changes may occur, the ovum 

 must, save in a few exceptional cases, undergo fertilisa- 

 tion, the essential feature of which is the union with it 

 of a second element, the sperm-cell. As a result of this 

 union, the cytoplasm of the two cells is condensed into 



