112 
MR. NEWPORT ON THE ORGANS OF REPRODUCTION, 
external semblance of a living animal ; but by the force of expansion of the growing 
body, the development of which being greatest along the dorsal, or larger curvature, 
exerts, in consequence, a greater degree of force against the middle of the dorsal than 
the corresponding part of the ventral surface ; the head and tail of the embryo acting 
as a fulcrum against the ventral surface only at the ends of the shell, and thus bend- 
ing it to the kidney-shaped form it assumes, while the dorsal surface of the embryo 
is gradually pressed through the opening. From the comparative rapidity of its en- 
largement immediately after the shell is fissured, it seems as if the stimulus given to 
it by exposure to a new medium, atmospheric air, is the great means of exciting its 
evolution. The embryo is now formed of eight distinct segments (fig. 5. a), inclu- 
ding the head, the ninth or anal segment being still indistinct. The head is more 
defined in its outline, and firmer in texture than other parts of the body, and is in- 
flected against the under surface of the prothorax (2.), or second segment, from which 
it is divided on the upper part by a deep transverse line. At its sides it exhibits a 
faint trace of the future antennae. The four thoracic segments also exhibit on their 
ventral surface little nipple-shaped extensions, three of which, on each side, are the 
rudiments of the future legs. When viewed from above, the body of the embryo 
appears compressed and wedge-shaped, its greatest diameter being in the second and 
third segments, while each succeeding segment is more and more contracted. I was 
unable at this period to detect any separate internal organs, the whole embryo being 
still a congeries of vesicles or cells, in the midst of which there seemed to be some 
faint traces of the commencement of an alimentary canal. 
It has already been stated, that certain markings exist on the egg at the time it is 
deposited. These are all on the external surface of the shell. They are seen, in the 
empty shell (fig. 5. b), to arise from some deficiency in the several layers of which 
it is formed. Some of these markings (c c) show that the shell is composed of at 
least four concentric layers or coverings, the deficiency in which, when viewed on 
the outside of the egg, presents an appearance which at first may readily be mistaken 
for the germinal vesicle and spot, seen through the shell. Each layer of the shell 
appears to be thicker the nearer it is to the surface. On the inner surface of the 
shell there is also often a slight marking ( d ), at that part which has not been sepa- 
rated, but forms a valve or connexion between the two halves. From the circum- 
stance of its being near the part where the funis joins the external membrane (5. a. e) 
that lines the shell, and is ruptured with it, there is some reason to believe that this 
may have been the situation of the germinal vesicle at the period of impregnation. 
The fissure in the shell, although always in its longitudinal axis, is often rough and 
uneven, and never entirely separates the two halves. 
At the end of the first day (fig. 5. a) I carefully removed the embryo and shell into 
diluted spirits of wine, and on examination beneath the microscope, found the body 
of the embryo covered with an exceedingly delicate cuticle, through which the cells 
it is formed of were distinctly visible. It was also completely inclosed in a smooth 
