CHAPTER XXI. 



EMBRYOLOGY. 



Develop- 

 ment is 

 from 

 simple 

 germs. 



Species 

 become 

 unlike as 

 their germs 

 develop. 



Develop- 

 ment is 

 differen- 

 tiation. 



IN tlie chapter on Organic Development it has been stated 

 that all development is from the simple to the complex. 

 Every organism is developed from a simple structureless 

 germ, and the germs and germinal matter of all organisms 

 are in appearance exactly alike ; there is no test, chemical 

 or microscopic, by which the germ, or small mass of 

 germinal matter, that is capable of developing into the 

 hio-hest Vertebrate can be distinguished from that which is 

 capable of developing into a worm. Consequently, if it 

 were possible to watch the development of a Vertebrate 

 and of a worm, or of any other widely separated species 

 of organisms, we should first see them perfectly similar, 

 and afterwards see them becoming gradually more and 

 more unlike, until their development was completed. 



It has also been stated that development essentially 

 consists in differentiation; that is to say, the process of 

 development consists in the increasing unlikeness of tissues 

 from each other, and the increasing separation of organs. 

 When development is watched under the microscope 

 (which can be done with many of those comparatively 

 low animals that have bodies composed of transparent 

 tissues, and with the eggs of fishes and frogs), the original 

 structureless and homogeneous germ is seen to transform 

 itself into different organs and tissues, each occupying its 

 own part of the body of the developing organism. 



From the truth that development consists in differentia- 

 tion, it follows that the greatest differentiation is the 

 highest development. Those organic species are the most 



