510 
MR. J. F. BULLAR OR THE DEVELOPMENT 
into a series of more or less distinct undulations. The lower layer of cells is thicker 
beneath the centre of each segment than between the segments, but it cannot be said 
to be distinctly segmented. At the posterior end of the body the cells are less 
numerous than at the anterior end. Some of these cells are slightly sunk in the yolk 
and appear to be isolated from the rest, but they may very possibly have been connected 
with them by means of other cells which lay outside the plane of the section, and 
which have thus been removed. They are not nuclei, but distinct nucleated cells . 
On the dorsal surface of the egg, towards the anterior end, the dorsal organ is seen 
in transverse section (fig. 7 d); it consists of a single layer of small cells, thickened in 
the middle, and causing a slight depression on the surface of the yolk. A series of 
flattened cells may be seen extending over the surface of the yolk for some distance 
from both the head and tail ends of the Keimstreif) and elongated nuclei occur at 
various points beyond them. 
T1 rough these cells do not appear to form a continuous 1 aver in any one section, yet, 
after the examination of a number of sections and from the appearance of the entire 
egg, I have no doubt that they do so in reality. 
Their non-appearance as a continuous layer in a single section is probably due to 
their extreme thinness and to the fact that, like the cells of the dorsal organ, they do 
not stain nearly so readily as the other cells of the embryo. 
In the next stage the inner membrane has become thicker and is less closely applied 
to the yolk. All the segments are now cpiite distinct except the seventh thoracic, 
which does not make its appearance till a later period. 
The direction of the telson (fig. 9, T) should be noticed. It is generally stated that 
one of the differences between the Amphipoda and Isopoda consists in the direction of 
the tail of the embyro, which is bent downwards in the Amphipoda and upwards in 
the Isopoda. Thus Fritz Muller (‘ Facts for Darwin,’ p. 71) says: “ The curvature of 
the embryo upwards instead of downwards was met with by me, as well as by 
IvATHKE, in Idothea, and likewise in Cassidina, Philoscia, Tancds, and the Bopyridee ; 
indeed, I failed to find it in none of the Isopoda examined by me.” And at p. 74 : 
“ The Amphipoda are distinguishable at an early period in the egg by the different 
position of the embryo, the hinder extremity of which is bent downwards.” If my 
figure (fig. 2, B, ivoodcut ) is compared with Van Beneden’s* figure of My sis, and 
with Bobretzky's! figures of Oniscus, we shall, I think, be able to see the way 
in which this difference of bending has come about. In My sis (A), the posterior end 
of the body is formed very early, and is bent sharply forwards. In Cymothoa (B), 
development proceeds gradually from before backwards, and the posterior end is the 
last part to appear ; it is bent forwards in precisely the same way as in My sis, but 
naturally forms a smaller protuberance as it here represents but a single segment (the 
telson), while in My sis it represents several. If the embyro My sis were drawn out as 
* “Development cles Mysis,” Bull. Acad. Roy. Belg., t. xxviii., 1869* 
f ZeiL fttr Wiss. Zool., bd. xxiv., ph xxii., figs. 15 and 17. 
