260 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
X. GENERAL MORPHOLOGICAL QUESTIONS. 
Concrescence . — The theory of His, that the vertebrate embryo is formed by the 
concrescence of two halves along the median dorsal line, has drawn many of the argu- 
ments used for its supiiort from the development of the Teleosts ; and in the study of 
any fish, the iiresence or absence of indications of concrescence must be looked on as 
one of the more important general questions involved. However attractive in the 
iibstract the theory may be, I have failed to find in the Bass development any facts 
which should induce one to accept it, and the arguments commonly used in its favor 
seem to be very far from conclusive. Indeed, the only good argument I know of is 
Eyder’s observation (35) that in Elacate the extra-embryonic germ ring gives indica- 
tions of being divided up into somites. But I do not think this point can be made 
much of until Professor Ryder publishes a more detailed account of the embryos he 
observed, for the exact relations of the several parts of the embryo at the tail end can 
scarcely be ascertained from the existing account. Henueguy has in his last pai)er 
(18) reviewed the arguments for the concrescence theory, and as I agree in the main 
with his criticism it is unnecessary for me to recapitulate them. I will therefore sim- 
ply describe the growth of the Bass embryo. 
In the^growth of the blastoderm round the yolk, the head end of the embryo does 
not remain a fixed point, the body lengthening in an autero-posterior direction, as 
His supposed. On the contrary, the tail end of the embryo (posterior pole of the 
blastoderm, j?. p.. Fig. 35, 36, and 38, PI. xoii) remains a comparatively fixed i)oint, as 
Oellacher first showed, while the anterior pole of the blastoderm travels l apidly round 
the yolks (arrows. Pigs. 35 and 36). The point where the blastopore closes is thus 
but a short* distance from the original position occupied by the posterior pole of the 
blastoderm. Owing to the constant iiosition of the single oil globule, these facts can 
easily be made out (compare Figs. 35, 36, and 38). 
The growth of the embryo itself is more complicated, but still susceptible of what 
seems an accurate analysis. On comparing Figs. 35 and 36, it is seen that while the 
liosterior imle of the blastoderm remains comparatively fixed, the head end {h, e.) of 
the embryo follows, though at a much slower rate, the anterior pole of the blastoderm 
in its growth round the yolk. The comparison of the two figures inevitably leads to 
the conclusion that the increase in length, which the embryo undergoes in passing 
from one stage to the other, is due to intussusception and not to concrescence. Ex- 
tending the comparison to the later stage (Fig. 38, PI. xcii, just before the blastopore 
closes) it is seen that the increase of length, which thp embryo undergoes between the 
stages represented by Figs. 36 and 38, is brought about in a different way from that 
between Figs. 35 and 36. This is shown by the following examination : At the begin- 
ning of the older period (Fig. 36) the head end and tail end of the embryo are approx- 
imately equidistant from the oil globule, and at the end of the period (Pig. 38) the case 
is the same. The head end of the embryo has therefore continued to grow round the 
yolk, as in the period Figs. 35 to 36, and the body has also been lengthened at the oppo- 
site end in the opposite direction. The increase in length at the tail end of the embryo 
deserves especial attention. The great increase in length, which the body undei’goes 
by the growth round the yolk of the head end of the embryo (Figs. 35 to 36, and also 
Figs. 36 to 38) can only be explained as ordinary growth by intnssusception. If this is 
so, it is perfectly fair to assume, until the contrary is proved, tliat the comparatively 
