DEVELOPMENT OF ANIMALS. 645 
side in bundles (e) ; from each one acilium or “ tail’? grows 
out, when they are set free from the mother-cell. In this 
tailed form they are very active, and effect the fertilization 
of the egg of an animal of the same species. This is due to 
contact of one spermatozoon with the female pronucleus situ- 
ated in the egg. Immediately after the spermatozoon has 
penetrated into the egg, its ‘‘head’’ is converted into a 
nucleus, called the male pronucleus ; after this, radiating 
strie make their appearance around its surface ; then the 
male pronucleus travels toward the female pronucleus, and 
finally the male and female pronuclei fuse together and form 
the first ‘‘ segmentation nucleus.’’ 
This nucleus subdivides, and the result is a mass of cells 
resembling a mulberry, and hence called the morula. The 
outer circle of the cells of the morula may hereafter form 
what is called the blastoderm ; after a while it pushes in at 
one point, and the portion thus forced is called the inner 
germ-layer (endoderm) and the outer is called the ectoderm 
or outer germ-layer, and in this condition the germ is called 
a gastrula. Subsequently, a third layer develops from the 
endoderm, which is called the mesoderm, and after this the. 
different tissues become developed. 
All animals, from sponges to man, become first two- and. 
afterward three-layered sacs ; so that all animals above the 
Protozoa not only, as a rule, originate from eggs, but may be: 
said to travel, up to a certain point, the same developmental 
path. From this point the members of different types of 
life diverge. How different are the modes of development 
of animals has been set forth in the different life-histories 
related in the foregoing pages of this book.* But the laws 
of growth are as stable and uniform—certain causes pro- 
ducing certain results—as the laws of the motions of the 
heavenly bodies. 
When the workings of these laws of development are in- 
terfered with by sudden accidents, by too scanty nourish- 
ment, and by the transmission of the effects of such acci- 
* For a fuller, more consecutive, though still fragmentary account, 
the reader is referred to the author’s ‘‘ Outlines of Comparative Em- 
bryology, or Life Histories of Animals, including Man.”’ 
