440 A. A. W. HUBRECHT. 
and active splitting of the mesoblast requires the demonstra- 
tion of anterior phases in which no such cavity was present, of 
the successive steps by which it gradually arose, and of the 
special adaptative significance of these transitory stages in 
each case. 
That the latter process is far more complicated and in many 
cases more unintelligible need hardly be insisted upon. More- 
over, the term “ schizocel,”’ introduced by Huxley, originally 
had amuch more definite and limited meaning than it has 
nowadays gradually acquired. Huxley’s own words are (‘ Quart. 
Journ. of Micr. Sc.,’ vol. xv, p. 54): “In the Schizoceela a 
perivisceral cavity is formed by the splitting of the mesoblast ;” 
and the same author is not unwilling to look upon the peri- 
visceral cavity of the Polyzoa as ‘“‘a blastoccele, more or less 
modified by the development of the mesoderm” (‘ Anatomy of 
Invertebrated Animals,’ p. 460). 
O. and R. Hertwig, in their contributions to our apprecia- 
tion of the nature of the different forms of ccelom, have applied 
the term schizoccel to the perivisceral cavity of many inverte- 
brate animals, and there is a strong tendency to apply that 
name to all such cavities that arise in what they call the 
“mesenchyma.” And when they come to ask, ‘* Wie verhalt 
sich das Schizocoel der Mollusken zu dem Blastocoel ihrer 
Larven? (‘ Coelomtheorie,’ p. 13) they immediately answer: 
«‘ Von Anfang an ist ein weites Blastocoel vorhanden, dessen 
Raum durch die zunehmende gewebebildung eingeschrankt 
wird. Die iberbleibenden spalten sind die ersten Anlagen 
des Schizocoels, dass sich nun secundaér wieder zu einem 
einheitlichen Raum gestaltet. Zwischen Blastocoel und Schi- 
zocoel wurde sich demnach eine ununterbrochene Continuitat 
nachweisen lassen.” 
The question, as we have formulated it above, whether the 
origin of this cavity must be looked upon as a passive phe- 
nomenon or as an active excision, originally adaptative and ren- 
dered permanent by heredity and selection, is thus in silence 
passed over by them. I have sufficiently insisted upon my 
reasons for keeping these two processes well apart ; and it will 
