1897] Cephalic Homologies. 933 
ticulum remains hollow throughout life; whether it is of en- 
todermal origin or not isunknown; Spengel considers it prob- 
ably ectodermal. A notochord is a band of entodermal cells 
differentiated along the dorsal median line and ending origi- 
nally at the blastopore, with the walls of which it is fused, and 
it lengthens by additions at its blastoporic end ; it is never 
hollow and never a canal. These characteristics of a notochord 
are invariable, but not a single one of them pertains to the 
alleged notochord of Balanoglossus. The question arises, if 
two organs are found to have no resemblance to one another, 
is their homology demonstrated? Surely, a morphologist can 
give but one answer. My second point touches upon Bate- 
son’s views concerning metamerism. These views are based 
on the tacit assumption that serial repetition of organs or parts, 
even if irregular, is the same as segmentation of the body. 
But embryology had demonstrated, before Bateson’s time, the 
true morphological basis of metamerism in segmented animals. 
This basis is the division of the mesothelium, as above stated, 
into paired symmetrical blocks. That this is the case is not 
an opinion, it is merely the summary statement of thousands 
upon thousands of direct observations. Serial repetitions, 
therefore, of ectodermal or entodermal organs, without these 
mesothelial segments, are morphologically not segmentation. 
Bateson’s assumption that there is, what we may name a 
“miscellaneous metamerism,” which, beginning in various parts, 
may lead on to true mesothelial metamerism is contrary to 
all morphological canons. That he considers this assumption 
necessary is another indication of the inherent weakness of his 
case. 
So far as knowledge and discussion have brought us to-day, 
morphology has the choice between the two theories as to the 
origin of Vertebrates, the Appendicularia and the Annelid 
theory. A decision between these two alternatives, which 
shall be convincing to zoologists, is not at present possible, yet 
something, I think, may be determined as to the probabilities 
of the final choice. : 
D. The Appendicularia theory is the direct outcome of Kowa- 
lewski’s embryological researches, and has been the subject of 
