278 The Unity of the Organism 



the Radiolaria. In. the general part, entitled Form and 

 Form-development in the Radiolaria, of his splendid report 

 on the collections of this group made by the German Deep- 

 Sea Expedition, not only do we find a section Ontogenesis of 

 Variations, but facts and questions of "ontogeny" are scat- 

 tered throughout the whole part, thus : "The other sort of 

 variability may be in large part referred back to changes 

 in some of the elementary processes which normally co- 

 operate in the ontogeny of the skeleton, especially to be 

 considered being the secretory and sprouting operations." 

 And Hacker laments that in spite of the great quantity of 

 excellently preserved material at his disposal he was unable 

 to work out the "complete embryology" (Entwicldungs- 

 geschichte) of a single group of Tripylea, though he was 

 able to add largely at many points to previous knowledge. 



From the standpoint of descriptive embryogeny probably 

 the most serious gap in our knowledge of the development 

 of the Radiolaria is the scarcity of information about the 

 growth of the adult animal from the swarm spores. Thanks 

 to several zoologists, among the latest of whom is Borgert 

 particularly, we know quite fully the mode of spore or 

 gamete production in several species. The great number, 

 small size, and simple structure of these germinal elements 

 in species like Aulacantha leave no doubt that their growth 

 to full si/td animals is an elaborate operation involving 

 many stages and kinds of differentiation. 



We have now gone far enough into the structure and de- 

 velopment of the higher protista to make it indubitable' 

 that there is a morphology, individual and comparative; an 

 embryology, also individual and comparative ; and a physiol- 

 ogy, likewise- individual and comparative, of this great sub- 

 division of the animal kingdom, not a whit less definite and 

 hardly less rich and varied, though less elaborate as to 

 individual animals than in the subdivision known as nietn/oa. 

 In the light of the great body of knowledge now available, 



