Rathke, Grmvth of the Eggs of the Crawfish. 249 



primae vie, commencing on the one hand with the oesophagus, 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 oesophagus 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 oesophagus. 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 well as 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 



