EMBRYOLOGY OF THE SEA BASS. 
261 
small increase in length, which the body receives at the tail end, is due to the same sort 
of growth. And I have no doubt that this is true of the greater part of the caudal 
increase. But the development, during the closure of the blastopore, of what has 
been called the primitive streak clearly leads to the conclusion that, as the blastopore 
closes, the germ ring is drawn into the tail end of the embryo, which is thus progres- 
sively lengthened, the final addition (of this sort) to its length being the incorporation of 
the secondary caudal mass, sec. c. ?»., Fig. 65, PI. xcvi. The formation of the teleos- 
tean primitive streak is obviously so similar to that of Amphibia (compare Schwarz, 
39) that the two must be regarded as homologous. In Triton (Hertwig, 
20) the blastopore closes in a slit-like fashion (see Fig. 7), leaving at its 
lower end a small opening, through which protrudes the clotterpro 2 )f. 
The line along which the blastopore closes is indicated after closure by 
a groove, p. g. Now compare with the diagram of the Triton blastopore 
the section given in Fig. 65, PI. xcvi. The primitive streak in the 
Teleost represents the line of closure in Triton; at the posterior end of 
the streak there is the same opening; and the opening is plugged u]) 
by a dotterpropf. The formation of the primitive streak in the Am- 
phibia, by a true concrescence of the blastopore lips, is undoubtedly 
the ancestral method, of which the j)rocess made use of in the Bass 
must be regarded as an embryonic modification. 
To sum up, the growth of the embryo takes place, as I believe, in 
the following manner: The great increase in length is acquired by the 
head end of the embryo following the anterior i>ole of the blastoderm in its growth 
round the yolk. The embryo is also lengthened in a much less degree by the movement 
of the tail end in the opposite direction. The growth in each of these cases is brought 
about by intussusception. The blastopore closes in a manner which is clearly a modi- 
fication of concrescence, and gives rise to the terminal portion of the embryo in which 
there is a median fusion of layers. 
Thecoucresceuce theory of His ce rtainly receives no confirmation in the development 
of the Bass. All the facts regarding the growth of the embryo that I have observed 
are incompatible with it, the only part of the embryo which is formed by concrescence 
being the posterior end, behind Kupffer’s vesicle; and the increase in the length of 
the embryo, by the addition to it of the primitive streak, is obviously a secondary modi- 
fication resulting from the transformation of a holoblastic egg (like the Amphibian) into 
a meroblastic egg. This is made plain by an examination of Fig. 65, PI. xcvi. In the 
ancestral holoblastic embryo the xirimitive streak did not lie horizontally, but more or 
less vertically; i. e.. it represented the posterior end of an embryo, in which the yolk 
was comparatively small and went to form the ventral wall of the gut. With the for- 
mation of the large, x>urely nutritive, yolk, the posterior end of the embryo came to lie 
in a horizontal jilaue, and so added to the length of the embryonic body. 
Among recent advocates of the concrescence theory (Ryder, 34, 35; Cunningham, 
8) the closure of the blastopore in Teleosts has been regarded as affording strong 
evidence of the truth of the theory. But it seems to have been assumed, without any 
satisfactory grounds, that the tissue of the germ ring, as it is drawn into the embryo, 
comes to lie along the notochordal line. Cunn ingham’s argument in brief is as fol- 
lows : (1) the non embryonic part of the germ ring disa^ipears ; (2) it is not absorbed, 
and must consequently be drawn into the embryo ; (3) the latter is hence formed by 
-p.y. 
dp. 
Fig. 7. — Diagram 
ol' closing lilasto- 
poro ol' Triton. 
