FURTHER REMARKS ON THE CELL-THEORY. 335 
is only found in sexual reproduction, not in asexual. The 
characteristic of the unicellular form is its simplicity of struc- 
ture, and the essential feature of sexual reproduction is the 
conjugation of the reproductive cells. Now in the Protozoa, 
in which the amount of formed tissue is generally slight and 
the structure of the body simple, conjugation can and does 
often take place between the ordinary form of the species. 
But in the Metazoa, in which conjugation is as necessary a 
phenomenon in the specific cycle as in Protozoa, conjugation 
is impossible between adult or ordinary individuals of a species 
from mechanical causes. How is this difficulty got over in 
nature? My answer is, by the formation of special individuals 
of extremely simple structure—a structure so simple that conju- 
gation between them is possible. To put the matter in another 
way, I should regard the ordinary dicecious Metazoon as a tetra- , 
morphic species, consisting of male, female, ovum, and _ sper- 
matozoon, the two latter being individuals which are specially 
produced to enable conjugation to take place. 
Mr. Bourne, in his criticism, begins by complaining that he 
cannot ascertain from my article my own views on the subject 
of the cell-theory. Why should he expect or wish to discover 
them? My remarks were simply directed to show the short- 
comings of the theory with regard to certain anatomical facts. 
As explained above, my own view is that the ceil-theory is 
inadequate to explain the facts, and that it is not possible at 
present to explain them by any theory. He proceeds to state 
that Iam abusive because I say that certain observers “ are 
constrained by this theory with which their minds are saturated, 
not only to see things which do not exist, but also to figure 
them” (I am referring to embryonic mesoderm of verte- 
brates). He calls this abuse, not argument. I venture to 
differ with him—it is neither abuse nor argument ; it is merely 
a statement of fact (unless, indeed, it be considered abusive 
to say that a man accepts and believes in the cell-theory). If 
you disbelieve it, consult the memoirs of the last twenty years 
in which this tissue is referred to, and in most of them you 
voL. 38, PART 2.—NEW SER. Y 
