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 ali the ecelomic 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 
ecelom with its cells of various function. Thus, to quote again from 
Lankester, “The ccelomic fluid and the celomic 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 
