258 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
behind and in front of Fig. 95 the head mesoblast forms a compact mass. A section 
through the region of the branchial sense organ is shown in Fig. 96, PI. xoix, and on 
comparing this section with the earlier stage (Fig. 71, PI. xcvii) it is seen that the head 
mesoblast masses have grown forwards. Still farther in front (Fig. 97, PI. xcix) the 
mesoblast consists of scattered cells. 
The compact head mesoblast (head mesoblast masses) continues to increase in 
amount and grow forwards. At the time of hatching it extends up to the eyes. A 
series of sections (Figs. 127-134, numbered from behind forwards) through this stage 
will illustrate its condition. Fig. 128, PI. cm, is through the extreme anterior so- 
mite region. Compared with Fig. 127, it is seen that anteriorly the somites decrease 
greatly in size, and also that they come to lie beneath the medulla. Just in front of 
this section the somites come to an end and are followed by the head mesoblast masses, 
with which the body cavity is continuous (Fig. 129, PI. cm). Still further in front 
we come to the gill-slit region ; Fig. 130, PI. cm, is, on the right side, through the 
slit, and on the left side just behind it. Comparing this figure with corresponding 
sections through earlier stages (Figs. 95, PL xcix, 114, PI. ci), the marked increase 
of mesoblast underneath the foregut is noticeable. The successive sections in front 
of the foregut. Figs. 131, 133, and 134, sufliciently indicate the condition of this part 
of the head mesoblast. 
The pericardial cavity does not develop until after hatching. The cells which 
inclose it are split off from the under surface of the head mesoblast masses. 
The development of the head mesoblast in the Bass, from a few scattered cells 
which proliferate and give rise to compact masses, which gradually acquire a greater 
forward extension, is evidently an extreme case of coeiiogeny, and makes any attempt 
to study the early morphology of the head in such a fish as the Bass an almost hope- 
less task. 
Mesoblast of the pectoral fins. — The pectoral fins do not form protuberances until 
a couple of days after hatching, but the mesoblast which gives rise to them begins to 
accumulate in the last few hours of embryonic life. Ziegler (47) has described the 
mesoblast of the fins as in direct continuity with, and as derived from, the head meso- 
blast. In the Bass there is no direct continuity between the two. On the contrary 
the accumulation of cells which gives rise to the fins {pec. /., Fig. 136, PI. civ) is 
intimately associated with the body cavity. On passing forwards from the section 
represented in Fig. 136, the fin mesoblast comes to an end, the body cavity drawing 
away from the edge of the embryo and assuming an appearance about as shown in 
Fig. 127, PI. cm. Farther forward the head mesoblast begins. It is quite possible, 
however, that the lack of continuity with the head mesoblast is only apparent, and 
that cells may migrate (a few at a time so as to escape observation) from the head 
mesoblast to the position of the fins, though as to the probability of the supposition 
I have nothing to say. In a later stage (Fig. 139, PI. CY, pee. f.) the intimate associ- 
ation of the fin mesoblast with the body cavity still exists, and as before there is no 
direct connection with the head mesoderm. 
IX. HEART; AORTA; SUBNOTOCHORDAL ROD. 
Heart . — During embryonic life the heart consists of a flattened sac lying to one 
side of the median line, in the space between the mesoderm and periblast ( Figs. 133, 
Pl. CIV, and 151, PI. cyir, h). The sac is composed of cubical cells, and is open 
