IX THE HISTORY AND SCOPE OF ZOOLOGY 347 



logical relationships of the groups of the animal king- 

 dom was thus completed. Renewed study of every 

 group was the result of the acceptance of the genea- 

 logical idea and of the recognition of the importance 

 of cellular Embryology. On the one hand, the true 

 method of arriving at a knowledge of the genealogical 

 tree was recognised as lying chiefly in attacking the 

 problem of the genealogical relationships of the smallest 

 twigs of the tree, and proceeding from them to the 

 larger branches. Special studies of small families or 

 orders of animals with this object in view were taken 

 in hand by many zoologists. On the other hand, a 

 survey of the facts of cellular Embryology which were 

 accumulated in regard to a variety of classes within a 

 few years of Kowalewsky's work led to a generalisa- 

 tion, independently arrived at by Haeckel and myself, 

 to the effect that a lower grade of animals may be 

 distinguished, the Protozoa or Plastidozoa, which 

 consist either of single cells or colonies of equiformal 

 cells, and a higher grade, the Metazoa or JEnterozoa, 

 in which the egg- cell by " cell division " gives rise to 

 two layers of cells, the endoderm and the ectoderm, 

 surrounding a primitive digestive chamber, the archen- 

 teron. Of these latter I proposed to distinguish two 

 grades, those which remain possessed of a single 

 archenteric cavity and of two primary cell-layers (the 

 Ccelentera or Diploblastica) , and those which by 

 nipping off the archenteron give rise to two cavities, 

 the ccelom or body-cavity and the metenteron or gut 

 (Ccelomata or Triploblastica). To the primitive two- 

 cell-layered form, the hypothetical ancestor of all 



