164 GENERAL BIOLOGY 



with the unicellular zygote, which, following cleavage, 

 is changed into a blastula that, in turn, is transformed 

 into a gastrula. Both blastula and gastrula stages, 

 particularly the latter, may be very greatly modified 

 by secondary conditions. It was recognized by one 

 of the earliest embryologists that the younger stages 

 of all animals resemble one another much more 

 closely than do later ones. This generalization may 

 be stated : development is from the general to the 

 particular, from the relatively undifferentiated to the 

 specialized (Von Baer's law). The whole series of 

 transformations which an organism undergoes from 

 zygote to individual dissolution is termed its Ontogeny. 



Conjugation in the Protozoa. - The Protozoa, 

 in spite of their apparent simplicity, are in most 

 cases very highly specialized, and their specializa- 

 tion is nowhere so marked as in their methods of 

 sexual reproduction. The majority of them re- 

 produce rapidly under favorable circumstances by 

 binary fission, or, in some cases, by the formation of 

 spores. After a time, owing to influences which are 

 imperfectly understood, this asexual reproduction 

 slows down and comes to a standstill. It may be 

 artificially stimulated (in the case of ciliates) by 

 adding chemical substances to the medium, such as 

 beef-extract, potassium phosphate, etc. On the 

 other hand if each generation of the dividing ciliates 

 is segregated in a few drops of culture fluid, asexual 

 reproduction may go on apparently indefinitely. 

 Experimenters have followed such a line beyond the 



