EMBRYOLOGY OF THE SEA BASS. 
259 
along one side over the greater part of its extent. Posteriorly, however, it forms 
a closed tube (Fig. 117, PL cii, /(, arterial end of heart). The sac contains a plasma, 
which takes a light stain, and amoeboid cells. The heart begins to beat while in this 
condition. 
In regard to the origin of this simple embryonic heart and its further develop- 
ment I may say that my observations on the Bass differ in so many points from the 
careful accounts given by Henneguy (18), Ziegler (47), and Oellacher (33), of the for- 
mation of the trout’s heart, that I prefer reserving my description until I have had an 
opportunity of studying the process in the Salmonidee. 
Suhnotocliordal rod and aorta . — Before the alimentary canal closes in ventrally 
there is found lying above it a single row of cells {s. n. r., Fig. 76, PI. xcvii). The 
cells are flattened dorso- ventrally, and their i)osition with respect to the entoderm 
cells, from which it is sometimes difficult to distinguish them, makes it safe to con- 
clude that they are the uppermost cells of the enteric fold, which have separated from 
the rest of the entoderm. There is thus in the Bass a homologue of the entodermic 
subuotochordal rod of Selachians, as there is in the Trout (Henneguy). The further 
relations of the rod in the Bass are extremely complicated. It becomes intimately 
associated with certain cells which form the aorta, though I do not believe that it enters 
into the composition of the vessel itself. I have thought it worth while to set down 
my observations on the development of the aorta, though they do not lead to a con- 
clusion regarding the origin of the cells which form it. 
To the subuotochordal rod are added other cells which form a string, some three 
or four cells in section, a. an., Figs. 92 and 93, PI. xcix, and 103, PL c. The origin of 
these cells I do not know ; sometimes I have thought them hypoblastic, and again 
mesoblastic. The probability is undoubtedly in favor of the latter origin. Occasion- 
ally the new cells added take a shape and position precisely like the subnotochordal 
cells (Fig. 104); usually, however, they are of an irregular shape. During the forma- 
tion of this solid string of cells very few wandering cells are to be seen, though in a 
little later stage (Fig. 110, PL oi) they are conspicuous in the spaces between som- 
ites, chorda, etc. Whatever be the origin of this string of cells, it is a true “ aorten 
Strang” (Oellacher), in that it develops into the aorta. The cells, which compose it, 
separate so as to inclose, at first in an irregular fashion, a central space {aor., Fig. 110), 
the aorta cavity. In subsequent stages the bounding cells become flattened and com- 
pletely inclose the cavity. As I have said, I do not think the subnotochordal cells 
take part in forming the aorta, for even after it has become a perfectly closed tube 
they may sometimes be seen in their old i^osition just above the vessel. 
The atrophy of the postanal gut takes place from before backwards. As it pro- 
gresses, there is left in the jilace of the gut a string of cells (one or two thick), which 
constitutes the caudal part of the subnotochordal rod (Figs. 101 and 109, PL c; Fig. 
Ill, PL ci, s. n. r.). With this rod are associated some few amoeboid cells, the origin of 
which is unknown (Figs. iOl and 109), but which are jirobably concerned in the formation 
of the aorta in this region. The formation of the aorta in the tail, like the atrophy of 
the postanal gut and formation of the subnotochordal rod, progresses from before 
backwards. There is no previous formation of a solid string of cells. In the section 
Fig. 112, PL Cl, the subnotochordal cells, above the aorta, were unusually distinct. 
