THE PRINCIPLES OF EMBRYOLOGY A475 
present-day opinion, that the ccelom arose in the first place in the 
very closest connection with the germ-cells or gonads. Thus 
Lankester, in his review of the history of the ccelom, states :— 
“The numerous embryological and anatomical researches of the 
past twenty years seem to me to definitely establish the conclusion 
that the ccelom is primarily the cavity, from the walls of which the 
gonad cells (ova or spermata) develop, or which forms around those 
cells. We may suppose the first ccelom to have originated by a 
closing or shutting off of that portion of the general archenteron of 
Enteroccela (Ccelentera), in which the gonads developed as in Aurelia 
or as in Ctenophora. Or we may suppose that groups of gonad 
mother cells, having proliferated from the endoderm, took up a 
position between it and the ectoderm, and there acquired a vesicular 
arrangement, the cells surrounding the cavity in which liquid 
accumulated. 
“The ccelom is thus essentially and primarily (as first clearly 
formulated by Hatschek) the perigonadial cavity or gonoccel, and 
the lining cells of gonadial chambers are ccelomic epithelium. In 
some few groups of Ccelomoccela the cceloms have remained small 
and limited to the character of gonoccels. This seems to be the case 
in the Nemertina, the Planarians, and other Platyhelmia. In some 
Planarians they are limited in number, and of individually large size ; 
in others they are numerous.” 
When Lankester says that “the lining cells of gonadial chambers 
are ccelomic epithelium,” that is equivalent to saying that the lining 
cells of the ccelom form an epithelium which was originally gonadial, 
provided that, as seems to me most probable, his second suggestion, 
of the ceelom being formed from gonadial mother-cells which have 
taken up an intermediate position between endoderm and ectoderm 
and there acquired a vesicular arrangement, is the true one. It does 
not seem to me possible to conceive of the gonads arising from cells 
of the epiblast or of the hypoblast, in the sense that such cells are 
differentiated cells belonging to a layer with a definite meaning. 
When we consider that the gonad gives origin to the whole of a 
new individual, that in the protozoan ancestors of the Metazoa their 
ultimate aim and object was the formation of gonads, it seems a wrong 
conception to speak of the gonads as formed from cells belonging 
either to the gut-wall or to the external epithelium. The gonads 
must stand in a category by themselves; they represent a whole, 
