34 METAMOEPHOSES OF MAN 



over, the segmentation of the yolk, and formation of 

 the raspberry mass, cannot be due to the development 

 of a series of cells, as Schwann asserted; for we find 

 that, among certain mollusks and worms, the most 

 complete lobes of the vitelline structure occasionally 

 coalesce; thus showing that they are not provided 

 with any distinct covering.* 



Many serious mistakes, arising from the study of 

 the higher animals exclusively, have been corrected 

 by an anatomical investigation of marine invertebrata, 

 and we find that such an investigation is fatal to the 

 cell theory. It is quite true that this theory and the 

 facts to which it is applied, agree in many instances. 

 The majority of the mollusks, the Nemertes, Planariee, 

 Synhydrae, and almost all the creatures which I have 

 described elsewhere, f have skins of a more or less 

 decidedly cellular character ; and, moreover, all these 

 external tissues are primarily formed from cells. We 

 know this to be the case in membranes lining the 

 inner and outer surfaces of the various organs, but 

 we cannot say the same of the organs themselves. 

 We have never seen either muscular or nervous fibre 

 begin its development in the cellular condition, nor 



* I saw this for the first time in the eggs of Hermella. — (Ann. des 

 Sciences naturelles, 1848.) It has since been observed by others. 



f See the " Souvenirs d'un Naturaliste," 1842. Some of my 

 readers may think that I allude to this work oftener than is con- 

 sistent with modesty, but I know of no other in which this subject 

 is treated of in so popular a style. I cannot refer my present 

 readers to the "Annales des Sciences naturelles/' nor to the 

 similar publications in England and Germany I trust there- 

 fore that I may be pardoned for alluding to a work in which the 

 general reader will find much matter for which there is no space 

 in a volume like the present. 



