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- 

 ;ited 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 

 strias 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 th e 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." 



