Rathke, Growth of the Eggs of the Crawfish. 255 



volution of the sacculus in one direction only. The result of this dis- 

 tinction is, that the central parts of the nervous system are in Verlehrata 

 external, and in Annulosa internal, to the serous membrane. The 

 positions of the ganglionic cord corresponding with the spinal marrow, 

 and of the intestinal nerve corresponding with the great sympathetic, are 

 completely reversed in these two great divisions of the animal kingdom. 



A similar opposition occurs in the general position and direction of all 

 the external organs. In the Verlehrata, in Batrachia for instance, both 

 extremities, the head and tail, as soon as they become visible, expand over 

 the vitellus, and tend to inclose it. In the same manner, the legs and 

 arms, and in Birds the wings, embrace the vitellus, which is placed in 

 juxtaposition with the abdominal surface of the body; the primitive 

 portion of the embryo remaining at the back. In the Crawfish on the 

 contrary, the tail tends to become free, while the anterior extremity alone 

 is applied to the vitellus, the two ends approaching each other not by 

 surrounding the last named organ, but in the contrary direction. The 

 limbs too, instead of embracing the vitellus, take the opposite direction 

 and surround the outerside of the primitive portion of the embryo. 

 Hence it follows that in the Crawfish, and the same is the case in Spiders, 

 the inner side of the limbs corresponds with their outer side in Vertebrata ; 

 and that, when the Crawfish quits the egg, it turns the primitive part of its 

 embryo towards the earth, while the Verlehrata turn their'sin the opposite 

 direction. What is called the abdominal surface in the former is conse- 

 quently analogous to that which is termed dorsal in the latter; and 

 Annulosa turn their backs, while Verlehrala turn their bellies, towards 

 the surface of the earth. 



Other important points of distinction are found in the structure and 

 developeraent of the head and its parts, and in the disposition of the 

 brain and nervous system in general. But we cannot afford space to enter 

 further into these particulars. The authour concludes with some general 

 deductions, which are, however, sufficiently obvious not to require 

 repetition. Three plates, two of them accompanied by outlines, filled with 

 magnified representations of the egg and its contents during the various 

 stages of developement, and in various points of view, together with 

 their explanation at length, complete the volume. They are executed 

 with great care, and afford excellent illustrations of the growth of the 

 embryo in all its stages. 



