254 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
subsequent stages form the head mesoblast masses. The mesoderm plates give rise to 
somites and coelom. The forward coutinuatiou of the coelom (pericardial cavity) is 
derived from the head mesoblast, but does uot come into existence until after hatching. 
The formation of the head mesoderm and pericardial cavity will be considered later, 
and for the present the coelom, as contained in the trunk, will only be described. 
The formation of the coelom in the Bass takes place in a different way from that 
followed in the Trout. In the latter the embryo is not compressed laterally to such a 
great degree, so that the mesoderm plates lie in a more or less horizontal plane, and 
the coelom mesoblast is divided from the somites in much the same way as in a bird. 
In the Bass the great lateral compression leads to a deviation from this mode of form- 
ing the coelom, the deviation being probably a very common one amongst Teleosts. 
Three stages in the formation of the coelom are shown in Figs. 67, PI. xcvi (35 
hours), 75, PL xcvii (39 hours), 94, PL xcix (45 hours). In Fig. 67 (compare the earlier 
stage. Fig. 61, PL xcv) the embryo has already undergone great lateral compression, 
and the mesoderm plates are now inclined at an angle of 45° to the surface. There is 
already present in this stage a shallow longitudinal furrow which tends to divide the 
plate into two parts, the somite mesoderm [som.) and the coelom mesoderm (coel.). The 
furrow deepens as the next stage, Fig. 75, shows, and finally completely divides the 
plate into coelom and somites. Fig. 94. (It may be said here that during the formation 
of the coelom the somite mesoderm is at the same time dividing transversely into 
somites.) The coelom as thus formed consists of a two-layered idate of flattened cells. 
The layers are not sharply separated, and during embryonic life it is only at the inner 
angle, where the Wolffian duct is constricted off, that a true cavity appears between 
them. The condition of the coelom {coel.) at the time of hatching is seen in the succes- 
sive sections, Figs. 126, 127, and 128, Plates on and cm. Fig. 128 is the most anterior, 
and on comparing it with Fig. 94 it is seen that the diminution in size of the yolk has 
brought the coelom into nearly a horizontal plane. In the posterior part of the body, 
where the embryo has been entirely folded off from the yolk (Fig. 126), the lateral halves 
of the coelom have met beneath the alimentary canal, and the somatopleure and 
splauchnopleure are sharply marked off from each other. The coelom extends as far 
back as the anus, but at no time any further. 
During the first three days of larval life the coelom becomes transformed into 
something like its adult condition. A cavity aj^pears between the somatopleure and 
splauchnopleure, in the immediate neighborhood of the alimentary canal (Fig. 136, 
PL CIV, 86 hours). The coelom then extends beneath the canal (Fig. 138, PL civ, 
100 hours, and Fig. 139, PL cv, 112 hours) and the two halves ultimately meet, the 
ventral mesentery being absorbed. The gradual flattening which the cells composing 
the coelom wall undergo may be traced in the figures 136, 138, PL Civ, and Fig. 141, 
PL cv. The next step taken by the coelom is to grow down between the ectoderm 
and the yolk, on each side (Fig. 141, PL cv). By this time the liver has not only 
begun to develop (Fig. 138, 1) but has come to lie between the coelom and the yolk, 
and hence the coelom in its growth round the latter also envelops the liver. This is 
shown in Fig. 141, PL cv (136 hours) and in Fig. 145, PL cv (160 hours, ventral 
mesentery absorbed). When the two halves of the coelom eventually meet in the 
median ventral line beneath the combined mass of liver and yolk (at x in Fig, 145) 
another ventral mesentery will be established. But it is plain that the new ventral 
mesentery will be only a continuation of the one which has been formed and absorbed 
