( 23 ) 
Thus, along the surface which is to he the back of the chick, two 
longitudinal ridges appear, one on either side of the middle line, 
these thicken, become more elevated, converge, and finally join, 
enclosing a tubular cavity, which with its walls becomes the brain 
and spinal cord. Along the front, the two innermost membranes 
fold more and more inward until their two sides coalesce along 
the middle line, making a tube—the subsequent digestive canal. 
The two outer membranes also fold inward, but in a wider sweep, 
the two sides ultimately coalescing along the middle line, also, to 
make the general cavity of chest and abdomen, and, as will be 
understood through their broader sweep, enclosing the digestive 
canal/* Later, the protoplasmic cells constituting these mem¬ 
branes, tubes, etc., which, originally, w’ere precisely alike in ap¬ 
pearance and function, gradually become transformed in these two 
respects, according to their locality—becoming “specialized,” as 
we say. Thus, while every cell has all the properties of proto¬ 
plasm, one cell will show a higher development of one property, 
while a second will be specially characterized by some other at¬ 
tribute. For example, every cell of living-matter has the power 
of automatic motion, but the cell which forms part of a muscle 
has tliis very markedly developed. Again, every living cell is 
“irritable,” by which we mean having the power to react when 
anything comes into contact with it. its latent energy instantly 
being converted into an actual form ; this function is most marked 
in the cells of the nervous system. To present a brief statement 
of the transformation incident to the development of life in the 
higher animals, we may sav that by the multiplication of the cells 
which are the offspring of the original one , the formation of four 
membranes, the thickening and folding of these, and the special¬ 
ization of cell-functions, the complete animal is developed. 
The subject of the development of living things is an extremely 
complex one, requiring close study for its comprehension, and, in 
this evening’s lecture, with limited time, and few means of illus¬ 
tration at my command, I have not expected to carry you far into 
this most interesting study, but the broad principles of the plan 
according to which life developes are very simple, and I therefore 
•Figs. 13 and 13. 
