EMBRYOLOGY OF THE SEA BASS. 
251 
Beard {1. c.) sui)poses that the sense organs, primitively situated one above eac.h 
gill, gradually increased in number and spread over the bead. The phylogenetic 
method of increase he thinks was probably division, and in this I agree with him, 
basing my belief chiefly on the development of the lateral line organs in the Bass. In 
actual ontogeny the method generally employed would seem to be the formation of a 
sensory cord which grows out from a primitive center and along which sense organs 
develop by local iiroliferation (2). 
The serial homology of the nose, ear, and branchial sense organs, so strongly sup- 
ported by Beard, receives new confirmation in the development of the Teleost, thougli 
the original sac-like character of the branchial sense organ (lateral line organs also) 
in the ontogeny of the Bass makes it, I think, necessary to alter the details of the 
comparison instituted by Beard between these organs. Beard believed, and it is gen 
erally so believed, that the primitive condition of the segmental sense organ of verte- 
brates was that of a superficial sense patch, something like the lateral line organs of 
larval fishes. In the auditory and olfactory regions the originally simple sense patcli 
gave rise by division to a number of such organs, which were confined to a small area 
The whole area subsequently became invaginated to form respectively the auditory 
and olfactory organs, the sac- like structure of which is therefore highly secondary as 
compared with the superficial sense patch. Now, according to this view, it is difficult 
to account for the fact that the branchial sense organ and the (embryonic) organs oi’ 
the lateral line originate in the Teleost as sacs, which subsequently flatten out into 
the well known superficial sense organs. If the latter condition represents the prinu 
tive condition in the vertebrates, why need the organs go to the trouble of running 
through such a complicated metamorphosis'? It is, of course, impossible to reach a 
decision in regard to this point when the known facts are so few, but for the present 
it must be borne in mind that there is at least a possibility that the sac-like condition 
of the organs in the embryo Bass represents a phylogenetic stage; and in view of 
this possibility. Beard’s theory of the origin of auditory and nasal sacs from a collec- 
tion of segmental sense patches can not be accepted, for there is at least as much to 
be said for the other theory, viz, that the ear and nasal sac represent single sense 
organs. According to the latter view the nasal sac (which arises in the Teleost as a 
simple invagination) has retained more closely than any of the other members of the 
series the structure of the ancestral segmental sense organ ; the ear has been shut off 
from the surface and transformed into a closed vesicle, while the remaining organs 
have been flattened out into superficial sense patches. 
Lateral line . — Beard has described the formation of the lateral line in the Salmon 
(C) as takiTig place in the following way : In the region of the neck just behind the ear 
a cord of cells is split off from the nervous layer of the ectoderm; the cord grows 
backwards along the whole length of the embryo; it then becomes thickened in each 
segment of the body, the intervening parts growing thin and finally passing out of 
sight, though the author thinks they may still persist as flue nerve strands. The 
thickened parts becoifie the sense organs. The development of a sense bulb from one 
of the segmental thickenings takes place in the following manner: Certain of the 
cells which are next the outer surface lengthen until they reach the surface of the body, 
when they acquire terminal hairs; the remaining cells arrange themselves round the 
base of these cells as a center. 
