448 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
rating entoderm from ectoderm. There are in ail twenty-six sections in this egg 
(ovary 23); maximum diameter of egg, -272 mm. 
Of the second stage I have obtained several fair series of sections. The chorda 
is now well marked off for some distance and the mesoblast is split off from over the 
entire entoderm except at the caudal mass, where it is not possible to distinguish the 
three layers from each other. In all other regions the ectoderm is separated from the 
mesoderm by a large segmentation cavity. The sections, figs. 60 to 63, are parallel to 
a tangential over the caudal mass. 
Fig. 60 is the sixth section of the series and has not yet reached the yolk. The 
mesoblast is not distinguishable from the entoderm, owing to the fact, perhaps, that 
near the margin of the section the plane of the section is oblique to the plane sepa- 
rating mesoblast from entoderm. The embryonic region is indicated by the strand of 
cells between ectoderm and entoderm, and, in fact, by the whole of the central mass of 
cells. The grouping of the sex cells in this region is as usual in this stage (ovary 31). 
Fig. 61 is the eighth section of the series and the first which cuts tbe yolk. The 
entire outline of the notochord is evident at this place. The dividing line between 
•nesoderm and entoderm is not yet evident. 
Fig. 62 is the twelfth section of the series, and therefore near the middle of the 
egg. The greatest diameter of the egg is 3 mm. The line separating mesoblast from 
entoderm is here evident over the whole yolk; the chorda is a little thicker than the 
mesoblast on either side and touches the slightly thickened ectoderm above. The 
mesoderm is about two cells deep. 
Fig. 63 is the eighteenth section of the series. The thickness of both chorda and 
mesoblast is reduced. The ectoderm consists of an outer layer of flattened cells and 
an inner irregular layer of semicolumnar cells ; it also does in other sections, but the 
fact is not so apparent. It is not thickened over the chorda as far forward as this 
section. The chorda extends some distance farther, and appears finally to merge 
into the indifferent mesoblast seen along its sides in other regions. As far forward 
as I can trace it the entoderm forms a layer below it. It is very probable that the 
cells forming it are separated from the entoderm with the mesoderm by delamination. 
In that case a layer of entoderm cells would always be found below it. The chorda 
is formed some distance farther forward in another egg from the same ovary. In this 
egg the entoderm extends under the chorda as far as that structure can be followed, 
but near its end the sections become tangential again and the boundaries between 
tissues are not well marked. 
Fig. 65 is a section from the anterior portion of the chorda of this egg. The 
lower layer of ectoderm cells is in contact or approximated with the chorda for its 
entire length, and in the section figured the ectoderm is thickened and slightly 
depressed just over the chorda, the cells being somewhat radially .arranged. 
As stated above, in the caudal region the three layers merge into each other so 
they can not be separated (64). Another characteristic of the caudal region in all 
the eggs of this stage is the presence of numerous large sex cells. 
In the caudal region the neurula in eggs of this stage consists of the previously 
described thickened mass. In front of the caudal mass it rapidly narrows to a 
thickening about as wide as the chorda and immediately overlying it. The region 
above it is depressed into a perceptible shallow groove. Its lower surface (fig. 62) 
remains for some distance in contact with the chorda after quite a space is found 
