Rathke, Growth of the Eggs of the Crawfish. 249 
prime viz, commencing on the one hand with the cesophagus, which 
forms a union with the outer cavity of the mouth, and on the other with 
the intestine, connecting itself to its external opening beneath the tail. 
These two portions are soon after placed in continuity by the production 
of the stomach, After these parts the heart comes into existence, formed, 
as the authour believes, not from the internal layer, or mucous mem- 
brane, but from the outer or serous. It appears at first in the shape of a 
small compressed vesicle, seated near the junction of the anterior and 
posterior portions of the body. Several blood vessels are soon afterwards 
seen in its immediate neighbourhood, which may be regarded as prolon- 
gations of its substance; and its pulsation speedily becomes distinguish- 
able. About the same time appear the first traces of the nervous system. 
An elevation extends beneath the middle line of the central piece from 
the esophagus to the tail, with a slight longitudinal impression, and ten 
transverse superficial furrows dividing it into eleven processes, corres- 
ponding with the trophi and the legs, on either side. From these the 
muscles of those parts respectively take their origin. On the middle of this 
elevation is formed the ganglionic cord, consisting at first of eleven pairs of 
minute white spots ; and anterior to these a short and broad process passes 
forwards on either side of the cesophagus. Up to this period all the 
parts of the embryo, with the exception of the heart, blood-vessels, and ex- 
ternal parietes of the back, are formed of a uniform gelatinous transparent 
substance. The latter organs have more of a membranous consistence. 
In the fifth section the authour traces the progress of the embryo 
in its third period of developement, the termination of which is marked 
by the production of the salivary glands, occupying the remaining part of 
May. On this, as wellas on its fourth period, ending with the bursting 
of its envelopes and its escape from the egg, which is performed in the 
course of the succeeding. month, he enters into equally, or perhaps, 
owing to the greater distinctness of the parts, even more minute details 
than with respect to its previous developement. But our limits warn us 
that in spite of all our attempts to state his leading facts in as few words 
as possible, we have already encroached too much; it therefore becomes 
necessary to treat the remainder with the utmost conciseness. It may 
be sufficient then to say, that the whole of the organs developed during 
the foregoing periods continue, in the progress of these, to approach 
