HUXLEY AND HIS WORK. 767 



Haeckel has asserted that Huxley therein '-directed attention to the 

 very important point that the body of these animals is constructed of 

 two cell layers — of the ectoderm and endoderm — and that these, physio- 

 logically and morphologically, may be compared to the two germinal 

 layers of the higher animals" (Nature, 1874), and Professor Kowalevsky 

 has also claimed that Huxley "founded modern embryology by demon- 

 strating the homology of the germinal layers of vertebrates with the 

 ectoderm and endoderm of ccelenterates." (Nature, October 31, 1S95, 

 p. 651.) 



In all candor, I must confess that, important as the generalization of 

 Huxley for tbe Medusa? was, it was only applied by him to the Medusa?, 

 and was not necessarily extensible with the homologies indicated, but 

 it was pregnant with suggestiveness, and to that extent may have led 

 to the wider generalization that followed. Let all possible credit then 

 be assigned to it. 



The classification of animals generally adopted, and in this country 

 especially, up to at least the early years of the present half century, 

 was based on what was called plan or type, and was mainly due to 

 Cuvier. According to this school, there were four "great fundamental 

 divisions of the animal kingdom," and these were "founded upon dis- 

 tinct plans of structure, cast, as it were, into distinct molds or forras.' ; 

 The term generally used to designate this category was branch or sub- 

 kingdom, and the subkingdoms themselves were named vertebrates, 

 mollusks, articulates, and radiates. Various modifications of this sys- 

 tem and more subkingdoms were recognized by many zoologists, but 

 the one specially mentioned was in very general use in the United 

 States because favored by Agassiz, who then enjoyed a great reputa- 

 tion. Almost all naturalists of other countries, and many of this, 

 recognized the distinctness, as subkingdoms or branches, of the Pro- 

 tozoans and ccelenterates. But Huxley, in 1876, went still further and 

 segregated all animals primarily under two great divisions based on 

 their intimate structure, accepting for one the old name, Protozoa, and 

 for the other Haeckel's name, Metazoa. 



"Among those animals which are lowest in the scale of organization 

 there is a large assemblage which either present no differentiation of 

 the protoplasm of the body into structural elements, or, if they pos- 

 sess one or more nuclei, or even exhibit distinct cells, these cells do 

 not become metamorphosed into tissues — are not histogenetic. In all 

 other animals the first stage of development is the differentiation of 

 the vitellus into division masses, or blastomeres, which become con- 

 verted into cells, and are eventually metamorphosed into the elements 

 of the tissues. For the former the name Protozoa may be retained ; 

 the latter are coextensive with the Metazoa of Haeckel." 



While not exactly original with Huxley, the recognition of these two 

 great categories of the animal kingdom was hastened among natural- 

 ists, and found place in most of the works by men of authority that 



