1889.1 Days and Nights by the Sea. 413 
sometimes the whole egg with it, divides into 2, 4, 8, 16 parts 
in geometrical ratio. The resulting cells however do not 
separate as in the lowest forms of life, but remain united, and 
do not long continue alike but become dififere tiated. A very- 
complex physiological division of labor is finally established 
among them, and when the adult condition is reached, the 
body is a colony of probably many million of cells, constituting 
various tissues and organs, all of which work correlatively and 
harmoniously for the good of the whole. The adult healthy 
body may thus be compared to an ideal state, where the cells 
represent individuals or individual minds, all of which have 
the same faculties, although developed in different degrees. 
Yet all these subordinate units work together in a wonderful 
way for the good of a higher unit, the body or state. As the 
state has its executive and police officers to guard its interests 
and enforce obedience to its laws, so the body has the nerve 
cells of the nervous system, which in health regulate and 
coordinate the working of all the other organs. 
This fundamental conception of living things, known as the 
Cell Theory, was announced 50 years ago. It is no longer a 
theory but a fact, and from it every problem in biology must 
proceed. 
How then is it possible to follow these delicate and intricate 
processes by which the complex cell-state or community, 
which we call the animal, is developed from the egg ? The 
changes are chiefly internal, while the eggs, which are usually 
of microscopic size, are frequently opaque, and the protoplasm 
or Hving matter of the cells themselves, is colorless. Diffi- 
culties such as these, however insurmountable they may have 
been a generation ago, have been completely overcome, and it 
is now an easy task to divide an ^gg, which we will say is 
1-25 of an inch in diameter, or the size of a pin's head, into a 
series of 100 sections, each 1-2500 of an inch in thickness. 
These may then be placed in serial order on a strip of glass, 
and each of the 100 sections, which can now be studied with 
high powers of the microscope, is seen to be a picture in color, 
which plainly tells of the marvellous processes which have 
