EMBRYOLOGY OF THE SEA BASS. 
257 
sonaewliat dilfereut cliaractor, in that the muscle substance is not formed as separate 
fibers, but as (apparently) homogeneous masses. Duriug the first day of larYal life, 
the iieripheral part of the hitherto protoplasmic cell becomes transformed into three 
or four masses of muscle substance, separated by strands of j)rotoplasm (Fig, 137, 
PI. CIV, four muscle fibers from somites of Fig. 136). The protoplasmic strands 
meet in an axial remnant of protoplasm, in which is contained the nucleus. In a 
subsequent stage of development (Fig. 142, PL CV) the protoplasmic strands have 
disappeared and the amount of axial protoplasm at the same time has grown less, 
the great bulk of the cell having now become muscle substance. This is the condition 
of the fibers in the oldest larva I have studied. 
The “ intermediate cell mass ” to which the somite is said to give rise in some 
fishes (Trout, Oellacher, Heuneguy) does not exist in the Bass. Ziegler, who has care- 
fully studied this structure (47), did not come to a positive conclusion in regard to its 
origin, but found that after giving off a number of blood cells it formed the “ stamm 
veue” '(fused cardinals) in the Salmon. It is undoubtedly absent in many fishes: 
Serrami^s, Engraulis (Wenckebach, 43), Labrax (Ziegler), and, as Ziegler says, has no 
homologue in other vertebrate groups. In those fishes in which it is absent, the ves- 
sels elsewhere formed by it are probably formed by scattered cells, as is the case wi th 
the aorta in the Bass. Ziegler’s manner of looking at the structure commends itself; 
it is a part of the general Bildungs-gewebe (represented for the rest by wandering 
cells), which in some fishes early acquires an individuality as an aulage for certain 
great vessels (47). 
In the Bass, as in other Teleosts, almost the entire somite goes to build up the great 
trunk muscles. What relation the cells, which subsequently form the skeleton, bear 
to the somites I am quite unable to say. 
Mesoderm of the head . — Hoffmann (17, p. 26, 1883) says, without describing his 
observations, that the mesoderm of the head is undoubtedly segmented in the Tele- 
osts. I have, however, not found any trace of segmentation in this part of the meso- 
derm, as the folloAving description of its development will show. It is quite possible, 
however, that in this, as in some other respects, the small pelagic egg of the Bass is 
more secondarily modified than that of the Trout. 
Before the closure of the blastopore the mesoderm in front of the paired plates 
(somite mesoderm) consists of scattered cells, and the total amount of it is small, as 
may be gathered from the transverse sections (Figs. 62, 63, and 64, Pis. xov and xcvi.) 
The origin of this part of the mesoderm has already been described. The iiaired 
plates towards their anterior end dwindle in size, so that the traiisitiou from somite 
mesoblast to scattered mesoblast is not very distinct. When the somites begin to 
form, immediately after the closure of the blastoiiore, the scattered cells in the neck 
region (Fig. 62, PI. xcv) increase in number and form a moderately compact mass 
(head mesoblast mass) in front of the somites, which extends forwards as far as the 
auditory invagination, and in front of that is continued as a collection of scattered 
cells. The condition of the head mesoblast at this stage is gathered from the series 
of sections. Pig. 68, PL xovi (just in front of the somites). Figs. 69 and 70, PL xcvi 
(through auditory invagination), and Fig. 71, PL xcvii (through the branchial sense 
organ). 
Forty-five hours, Figs. 95, 96, and 97, PL xcix.— When the foregut closes in (com- 
pare Figs. 71, PL xcvir, and 95, PL xcix) it is surrounded by scattered cells, but both 
Bull. H. S. F. 0. 89 17 
