194 COMPARATIVE ZOOLOGY. 
ferentiations. It brings out first the profounder distine- 
tions, and afterward those more external. That is, the 
most essential parts appear first; e.g., the nervous system 
and skeleton precede the digestive apparatus. And not 
only does development tend to make the several organs of 
an individual more distinct from one another, but also the 
individual itself more distinguished from other individuals 
and from the medium in which it lives. With advancing 
development, the animal, as a rule, acquires a more spe- 
cific, definite form, gains the ability of maintaining a tem- 
perature of its own, and increases in weight and locomo- 
tive power. Life is a tendency to individuality. 
Development of a Hen’s Egg.—The first change is 
the segmentation of the formative part of the yolk (germ- 
yolk) by a process of self-division. It separates into two 
spheres, which subdivide into four more, and so on till 
the whole is broken up into a myriad of cells. These 
cells finally arrange themselves into a layer (called dasto- 
Fic. 160.—First Stages in Seementation of a Mammalian Eee: A, first division into 
halves, with spermatozoa around it; Band C, progressive subdivision, ultimate- 
ly transforming the vitellus, or yolk, into a ‘‘mulberry mass” of globules, or em- 
bryo-cells. 
derm), lining the vitelline membrane; and a round light- 
colored disk in this layer is the germinal spot, or cicatric- 
ula, already mentioned. This is the first trace of organ- 
ization. Soon the germinal disk thickens, and splits into 
three layers: out of the upper one are ultimately formed 
