SOME PROBLEMS OF COELENTERATE ONTOGENY 535 
during early cleavage, or even later the retention of infusoria, as 
claimed by Merejkowsky (’83), though this may be doubtful.? 
Hence the facts herein adduced, together with the further fact 
of its extreme variation as to size, shape, position, or, still more 
significantly, its absolute absence in a large proportion of the 
species of the entire phylum, afford ample warrant for the con- 
clusion that, so far from having any necessary morphogenic or 
ontogenic significance, the blastocoel may be said to be absolutely 
devoid of anything of the sort, least of all of any relation to 
phylogeny. 
e. Cleavage homology. With the later development of the 
doctrine of homology there came to be involved varying phases 
of embryology, as shown above. One of its latest aspects is 
that concerned with cleavage, which has assumed a place of com- 
manding influence within recent years, as expressed in the flood 
of literature which sprang into existence dealing with the subject 
from every point of view,—normal, artificial, experimental. 
Conklin (’97), has stated well the subject as follows: 
In the whole history of the germ-layer theories I see an attempt to trace 
homologies back to their earliest beginnings. This problem is as impor- 
tant today as it ever.was, and whether one find these earliest homolo- 
gies in layers or regions of blastomeres or the unsegmented ovum itself, 
the quest is essentially the same. Within this question of the earliest 
homologies is included another of great and present interest, viz., the 
significance of cleavage. 
With the broader implications and relations of this subject 
there is neither the time nor occasion for extended review in 
2 T can but express the strong conviction that those who contend for the presence 
in such eases of a definitive segmentation cavity and blastula are in serious error. 
It seems not at all adequate to aver that the absence of any true blastocoel is due 
to the ‘abbreviation of this stage of development,’ as G. T. Hargitt (’09) has des- 
ignated it. As suggested above, but for the earlier theoretical significance 
involved in the matter, it may be doubtful whether any such contention would be 
made as that under review. To the writer it seems a pity to waste words over the 
subject in the form of argumentation. The facts are their own best exponent, 
and with these clearly apprehended there ought to be small occasion for contro- 
versy. The presence or absence of syncytial conditions has nothing whatever to 
do with the problem. Long before a syncytium has developed the morula has 
arisen as shown above, a fact as incompatible with the blastula as the planula is 
independent of the gastrula. 
