88 ADAM SEDGWICK. 



implies that the adult cells are derived from embryonic cells 

 which have been directly produced by the division of the 

 ovicell, does not apply to the embryos of Peripatus." 



In the days when these words were written it was a general 

 belief among leading histologists and physiologists that the 

 connections which were known to exist in some cases between 

 adult cells had arisen secondarily, and that the primary con- 

 dition brought about by the cleavage of the ovum was a com- 

 plete separation from one another of these units, of which 

 the body was supposed to be composed. There has been, no 

 doubt, a change of opinion since those days, and although 

 many biologists would still maintain that cleavage is complete 

 and results in the formation of separate units which later 

 become connected, there is a constantly increasing number 

 who would consider themselves misrepresented if one imputed 

 to them this belief not long ago universal, and the belief which 

 was supposed to follow from it, that the first stage in the 

 evolution of the Metazoa was a colonial Protozoon. But, as 

 I have said, opinions have changed since those days, and I 

 quote my words, written then, to show that I have long held 

 the view which I am now expressing, and that I was among 

 the first to attack a theory which had even then passed its 

 stage of usefulness, and is now holding men's minds in an iron 

 bondage. For although opinions have changed on this im- 

 portant subject, and although there are some who think that 

 they have escaped from the domination of this fetish of their 

 predecessors, yet as a matter of fact the cellular theory of 

 development is still rampant, still blinds men's eyes to the 

 most patent facts, and still obstructs the way of real progress 

 in the knowledge of structure. 



In order that I may not be met with the statement that 

 such a state of things exists only in ray own imagination, that 

 I am putting up a dummy merely to knock it down again, it 

 is necessary that I should give some proof that this hypothesis 

 has still the power which I ascribe to it. What is the cellular 

 theory of development? I am not concerned with what its 

 authors held ; what we want to know is, what is the present 



