136 PATTEN. [Vou. XII. 
has apparently persisted as a large, conical lobe, projecting almost vertically 
upwards from the surface of the yolk. It is evidently the same kind of a lobe 
seen in Figs. 13, 48, and 49. The body of the embryo, as in the three preceding 
figures, is reduced to a mere sac. The opening of the sac is triangular, and almost 
as wide as the sac is broad. A little to one side, on the floor at the anterior end, 
is a small, tubular invagination, oesophagus (?), directed vertically downwards. 
Fic. 83, X 60, sectioned. This is another important embryo. The meso- 
dermic area is relatively small, and not very sharply defined anteriorly, owing 
to the small number of mesoderm cells contained in it. Posteriorly, and 
this is a point of value in determining the poles of the embryo, the meso- 
dermic margin is decidedly thickened, and shows very clearly the effects of 
concrescence, f.a.c. The axial ectoderm shows the usual thickening, and is 
invaginated to form a complicated, thick-walled sac, opening by an oval aperture. 
The sac projects forwards beneath the ectoderm, and at its anterior end expands 
into two transverse tubes. If the nerve-cord and the cephalic lobes are still 
present, although they cannot be recognized as such, it is obvious that the floor 
and sides of the sac will consist of these organs, while the lateral eyes would be 
situated somewhere near the end of the lateral diverticula. This condition is 
what normally obtains in vertebrates, and is of obvious significance in view of 
the other resemblances which we have pointed out elsewhere, between the normal 
brain and eyes of Limulus and those of vertebrates. Such embryos are extremely 
rare, and may have no morphological value. But the possible significance of such 
variations should not be overlooked. 
Beneath the aperture of the sac, the floor of the same suddenly slopes down- 
ward, and at the bottom of this depression is still another smaller and tubular 
one that projects downward and forward into the yolk. The character of the 
tissues, which seem to be perfectly normal, and further structural details, are 
shown in the three transverse sections (Pl. X, Figs. 83',”,%). 
Fic. 84, X 60, sectioned. Of the mesodermic area, which is nearly circular, 
only the posterior portion is represented. The whole area consists of a thick layer 
of rounded, and often isolated, lymphoid cells, lying in a space between the yolk 
and a thin layer of ectoderm. ‘There is a prominent marginal thickening extend- 
ing completely around the mesodermic area. In the centre of the latter the 
ectoderm is thickened and deeply invaginated to form a roughly cubical cavity, 
partly closed over in front by a backwardly directed lip, or fold, and behind by a 
similar one directed forwards. 
The thick walls of this cavity probably represent that part of the embryo from 
which the nerve-cords and appendages are developed, the whole complex mass 
being reduced to a continuous, undifferentiated layer. 
From the anterior wall of the cavity projects a short, tubular outgrowth that 
looks very much like an embryonic stomodaeum, the resemblance being increased 
by the presence of a pair of muscle strands, which extend laterally from the tip of 
the tube to the surface ectoderm. The presence of this specialized tissue offers a 
sharp contrast to the very early embryonic character of the rest of the embryo. 
This fact supports my view, if further evidence were necessary, that these are 
very old embryos whose organs have been reduced, with this exception, to the 
simplest embryonic tissues. 
Fic. 85, X 60, sectioned. The mesodermic area here is small, circular, and 
poorly defined. The margin, however, at the anterior border is greatly enlarged 

