REPRODUCTION AND DEVELOPMENT. 505 



can bring on differentiation, and, through it, call into existence new func- 

 tions, and thereby forever determine a new course, through which it must 

 pass. It is because a due weight has not been given to this considera- 

 tion that many physiologists have depreciated the influence of external 

 circumstances, or even denied it altogether, for they have assumed that, 

 since we can not produce a more marked change than we do in the way 

 of accomplishing a variation in species by artificially altering the condi- 

 tions under which they exist, such conditions can have had but little 

 power in bringing them to their present state. 



Upon the whole, there can be no doubt that differentiation will occur 

 in a more marked manner according as the exciting impres- Organic chan- 

 sion is made at an earlier period of the organic career. Con- S^^/finft 

 versely, the more advanced the organism, the less the prob- periods of life, 

 ability of differentiation. For this reason it is that striking changes of 

 this kind are rarely witnessed in individual life : they occur chiefly in 

 the first embryonic states, and therefore, for the most part, require for 

 their full manifestation generation after generation. Great organic varia- 

 tions are not, then, to be expected in the individual, though they may be 

 distinctly manifested in the course of time by the race to which it be- 

 longs. 



CHAPTER IV. 



OF REPRODUCTION AND DEVELOPMENT. 



Relation of Organic Beings : they come from a similar Cell and develop to different Points. 

 Their Division by Classification is fictitious.- Development and Differentiation. Homogenesis 

 and Heterogenesis. They depend on physical Conditions. The reproductive State closes De- 

 velopment. 



Development is from the General to the Special. Law of Von Bar. Invariable Sequence in 



OF REPRODUCTION: 1st. By Generation. Conjugation and Filaments. The Sperm-cell: its 



Production. Spermatozoa. The Germ-cell: its Production. 

 Ovum in the Ovary. Its Structure. Corpus Luteum. 



Ovum in the Oviduct. Mulberry Mass. Germinal Membrane. The CJiorion. 

 Ovum in the Uterus. Membrana Decidua. Placenta. Development of the Embryo. Types 



of Nutrition. Of Conception. Of Gestation. Of Parturition. Influence of both Parents. 

 2d. By Gemmation. Budding of Plants and Animals. Of Grafting. Limit of Gemmation. 



Influence of Temperature on Gemmation. 

 Alternations of Generation. Its Explanation. 



IN the popular view of the organic world, each individual being is re- 

 garded as maintaining an existence independent and irre- Popular view 

 spective of all others, or, at most, only connected with those pJSenSXfo*. 

 of its own race or kind. Without any apparent disturb- ganic beings. 



