EMBKYOLOGY OF THE SEA BASS. 
239 
be overrated, for it would then give us a clear insight into that most interesting 
, problem: what is the mechanism by which heredity works in the ontogenetic forma- 
tion of organs ? Accepting the explanation, the formation of a groove could be con- 
ceived of as taking place, because a certain hypertrophic stimulus had been a])plied 
to a long narrow area of the membrane. To give the stimulus was the duty of 
heredity, but after that heredity had no need to concern itself, for the mechanical 
pressure set up by the growth of the cells effected the rest. 
In analyzing the “ principle of unequal growth,” we must give as great a precis- 
ion as possible to the facts of the case. What we really see is (1) that cell multiplica- 
tion takes place over the area w to w, and (2) that the area inyagiuates to form a 
groove. While it is possible that the second fact may be the mechanical necessity of 
of the first, it is also possible there may be no causal connection between the two. In 
such a problem as this, in which experimentation is out of the question, our only hope 
of a solution lies in a comparison of several cases where the result is the same, but 
where the two factors bear different relations to each other. Now, in the Teleost the 
factors, which in the invaginate ancestor were supposed to be causally connected, are 
wide apart in time. The cell multiplication takes place very early, and long after that 
is over the cells begin to move along lines, in general identical with those which in 
the ancestor marked out their path of progression. It wmuld thus seem probable that 
in this case (that of the Teleost) heredity deals directly with the individual cells, though 
by what means a cell is induced to travel from the periphery of the axeuplatte towards 
the axis, and is then sent out along the old lines, is assuredly' beyond our iiresent 
comprehension. 
Eeturning to the ancestral type, it is clear there is not only no necessity of apply- 
ing the “principle of unequal growth,” but the probability is that the same relation 
between heredity and the individual cells which was deduced in the case of the 
I Teleost exists here also ; and that, as the new cells are brought iuto existence by fis- 
j siou, they stick together and move along their paths not because of a mechanical pres- 
I sure but in obedience to what for want of a better term might be called the instinct of 
heredity. To sum up the case: When an area invaginates we get the impression that 
’ it does so because pressure is applied in a i)articular way. But the impression is due 
to the facts (1) that the cells migrate altogether aud (2) that cell migration and fission 
take place coincidently. And this view of the case is borne out by the observation 
that cells follow the ancestral lines of migration when they are clearly not under the 
influence of a common pressure. 
In this connection the peculiar modification of the folding process by which the 
alimentary canal of the teleost is produced, deserves a word. The manner in which 
the cells slip one under the other to take their place in the wall of the canal suggests 
strongly the comparative independence of the individual cells, and yet the result is the 
same as if a membrane had folded in the ordinary way. 
Eeturning to the actual development of the neural cord, it is dififlcult to assign the 
proper significance to the neural furrow, n./. It is clearly not homologous in the 
ordinary sense with the medullary groove of other vertebrates, for it disappears while 
the groove becomes the central canal. A careful examination of the surrounding cells 
(Fig. 57, PI. xcv) leads to the conclusion that the furrow owes its existence to the 
fact that these cells share in the general movement indicated by the arrows. This 
movement we have seen represents the ancestral invagination, and thus, though the 
