1 8 BIOLOGICAL LECTURES. 



experiment in biology as is sometimes assumed; neither method 

 can arrogate to itself a monopoly of certitude regarding facts 

 or causes. In the solution of the problems of development 

 both observation and experiment are necessary ; each has its 

 advantages and its disadvantages and one is no less important 

 than the other. 



I. DETERMINATE CLEAVAGE. 



Without attempting any final and elaborate definition of so 

 general a term as development, we may for our present purposes 

 say that it is progressive and coordinated differentiation. In 

 all Metazoa and Metaphyta the stages immediately following 

 fertilization are characterized by the cleavage of the egg into a 

 considerable number of cells. The question at once arises as 

 to the relation between cleavage and differentiation. Is differ- 

 entiation manifested in the cleavage of the egg ? Is there any 

 causal relation between cell-formation and differentiation ? 



There is abundant evidence that there is no necessary relation 

 between the two. Many instances of differentiation without 

 cell-formation might be given, e.g., many Protozoa, Protophyta, 

 the spermatozoa and ova of certain animals, intracellular differ- 

 entiation of many tissue cells, etc. On the other hand, cell- 

 formation may occur without differentiation, e.g., all ordinary 

 divisions of tissue cells and many divisions of embryonic cells. 

 When the two processes are related we may have: (i) cell- 

 formation following the lines of preceding differentiation, e.g., 

 certain cleavages of ctenophores, mollusks, and ascidians; or 

 (2) cell-formation and concomitant differentiation, e.g., many 

 cleavages of turbellarians, nematodes, annelids, and mollusks; 

 or (3) differentiation following the lines of preceding cell- 

 formation, e.g., many cleavages in the eggs of annelids, mollusks 

 and probably many other animals. 



In that pioneer work on developmental mechanics (Unsere 

 Korperform, 1874) Wilhelm His propounded the doctrine that 

 the organs and parts of an embryo are represented in the early 

 stages of development, perhaps even in the unsegmented egg, 

 by definitely localized germs (Anlageri). " The principle, ac- 

 cording to which the germinal disk contains the preformed 



