% 
250 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
postaual organs is probably a remnant of the cavity of the original lateral line anlage, 
and the manner in which the organs are formed appears to be essentially like that 
pursued in the case of the anterior organs; but henceforth whatever new organs are 
formed must follow a different course of development, for the cavity of the original 
anlage has, so to speak, been used up, and the posterior part of the sense tract is 
now a simple cellular cord without any cavity at all. It would seem probable that 
this terminal sense cord continues to grow backwards, developing sense organs along 
its course by local proliferation. The probability rests on the presence of a terminal 
simple cord iu the tail, on the existence of sense cords in the head, and on the for- 
mation of sense organs along such cords in Amia (2). 
The lateral branch of the vagus, which innervates the lateral line organs and which 
is so evident iu Selachian and (Hoffman, 21) Trout embryos, can not be distinguished 
iu the Bass during embryonic life, nor could I make it out iu larvae of 2 or 3 days. 
COMPARATIVE, 
Common sensory anlage . — As far as 1 know, the formation of the ear, lateral line 
anlage, and branchial sense organs, by the division of a common anlage, has never 
been recorded before. I was consequently pleased to find a couple of figures in one 
of Kupffer’s papers (26, 1884) which strongly suggest that the organs are formed in 
the same way in the Trout ; Kupff'er’s figures, which are surface views (Taf. 11), show 
that at first there is ou each side of the neck one sac, then two, and then three. The 
structures appear in the figures to be hollow, but Kupffer says they contain a central 
mass of loosely connected cells, Kupffer describes the structures as branchial arches 
aud says the ear arises independently of them. Unfortunately no sections are figured 
iu the paper, and it is therefore impossible to decide whether Kupffer is right iu his 
interpretation or not. The general resemblance of the “schlundbogen” in his Fig. 14, 
Taf 11, to the series of sense organs in my Fig. 148 is certainly very marked. 
The fact that there is in the Bass a common anlage for the ear, branchial sense 
organ, and lateral line has certainly no phylogenetic significance. It can only be re- 
garded as a convenient method of forming these organs, which the embryos of certain 
animals have adoi)ted. It however serves to emphasize in a striking way the serial 
homology between the organs which previous work has already made so probable. 
Branchial sense organ . — The existence of a histologically differentiated sense organ, 
which bears such obvious relations to the gill slit as does that of the Bass, makes one 
wish that the fate of the so-called primitive branchial sense organs of Selachian em- 
bryos were better known. In spite, however, of our ignorance regarding the precise 
fate of these organs, it seems impossible to avoid the homology between what I have 
called in the Teleost a branchial sense organ aud the patch of thickened ectoderm 
which Beard (5) describes above each gill slit in the Selachian. There also seems good 
reason for a(;cepting Beard’s belief that these “primitive branchial sense orgaus” in- 
dicate the position of a series of sense orgaus of very ancient origin. We are thus led 
up to the rather surprising conclusion that while in the Selachians the position of these 
ancient organs is only indicated by thickened patches of undifferentiated ectoderm, iu 
the highly modified group of Teleostei one at least of the organs has been retained in 
a functional condition, though probably very different iu structure from the ancestral 
organ. 
