476 THE ORIGIN OF VERTEBRATES 



while the other cells represent only a part ; they cannot therefore be 

 derived from the latter. They may, and indeed do, give rise to cells 

 of a subordinate character, but they cannot rightly be spoken of as 

 derived from such cells. The very fact mentioned by Lankester, that 

 in the lowest ccelomatous Metazoa, the Platyhelminthes, the cceloms 

 are limited to the character of simple gonoccels, strongly points to 

 the conclusion that all the ccelomic cells were originally of the nature 

 of gonadial cells, and therefore free-living and independent of the 

 rest of the cells of the body. Whether the germ-cells appear, as in 

 Hydra, to be derived from the ectoblast, or, as is usually stated, from 

 the endoblast, in neither case ought they to be classed with the internal 

 or external epithelium ; they are germ-cells, and the epithelium which 

 they form is neither epiblastic nor hypoblastic, but germinal, forming 

 originally a simple gonoccele, afterwards, in the higher forms, the 

 ccelom with its cells of various function. Thus, to quote again from 

 Lankester, " The ccelomic fluid and the ccelomic epithelium, as well 

 as the floating corpuscles derived from that epithelium, acquire special 

 properties and importance over and above the original functions 

 subservient to the maturation of the gonadial cells . . . the most 

 important developments of the ccelom are in connection with the 

 establishment of an exit for the generative products through the 

 body-wall to the outer world, and further in the specialization of 

 parts of its lining epithelium for renal excretory functions." 



Such exits led very early to the formation of ccelomoducts, which 

 are true outgrowths of the ccelom itself (p. 14) : " The ccelomoducts 

 and the gonoccels of which they are a part, frequently acquire a renal 

 excretory function, and may retain both the function of genital con- 

 duits and of renal organs, or may, where several pairs are present 

 (metamerized or segmented animals), subserve the one function in 

 some segments of the body, and the other function in other segments." 



The origin of the ccelom and its derivatives from a germinal 

 membrane, as suggested by Lankester, appears to me most probable, 

 and, if true, it carries with it conclusions of far-reaching importance, 

 for it necessitates that all the cells which line true ccelomic cavities, 

 and their derivatives, belong to the category of free-living cells, and 

 are not connected with the nervous system. The cells in question 

 are essentially those which line serous cavities and those which form 

 excretory glands such as the kidneys. In the latter organ we ought 

 especially to be able to obtain a clear answer to this question, for is 



