348 THE HISTORY AND SCOPE OF ZOOLOGY IX 



Metazoa or JEnterozoa,TLeiQckel gave the name Gastrcea; 

 the embryonic form which represents in the individual 

 growth from the egg this ancestral condition he called 

 a "gastrula." The term "diblastula" has more recently 

 been adopted in England for the "gastrula" of Haeckel. 

 The tracing of the exact mode of development, cell by 

 cell, of the diblastula, the ccelom, and the various 

 tissues of examples of all classes of animals has been 

 pursued during the last twenty years with immense 

 activity and increasing instrumental facilities, and is 

 still in progress. 



Two names in connection with post -Darwinian 

 taxonomy and the ideas connected with it require 

 brief mention here. Fritz Muller, by his studies on 

 Crustacea (Fur Darwin, 1864), showed the way in 

 which genealogical theory may be applied to the 

 minute study of a limited group. He is also respon- 

 sible for the formulation of an important principle, 

 called by Haeckel " the biogenetic fundamental law," 

 viz. that an animal in its growth from the egg to the 

 adult condition tends to pass through a series of stages 

 which are recapitulative of the stages through which 

 its ancestry has passed in the historical development 

 of the species from a primitive form ; or, more shortly, 

 that the development of the individual (ontogeny) is 

 an epitome of the development of the race (phylogeny ). 

 Pre-Darwinian zoologists had been aware of the class 

 of facts thus interpreted by Fritz Muller, but the 

 authoritative view on the subject had been that there 

 is a parallelism between (a) the series of forms which 

 occur in individual development, (b) the series of 



