THE PRINCIPLES OF EMBRYOLOGY 475 



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 Ctenopkora. 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 Xemertina, 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 ccelom 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, 



