Rathke, Growth of the Eggs of the Crawfish. 251 



We must here interrupt our analysis for a moment to notice the obvious 

 discrepancy between the facts detailed in the present publication, and the 

 theory advanced by Mr. J. V. Thompson with respect to the metamor- 

 phosis of Decapod Crustacea, of which some account is given at p. 248 

 of our last volume. The observations of Dr. Rathke prove beyond all 

 question that no such metamorphosis takes place in the young of the 

 Craw-fish, and thus confirm the doubts which we have there expressed of 

 the universality of the fact. We cannot, however, discredit Mr. Thomp- 

 son's statement that he has seen the ova of the Common Crab give birth 

 to animals of a form very different indeed from that of their parent; we 

 will therefore only observe in conclusion, that if there existed no optical 

 delusion or other cause of error in the isolated observation which he has 

 given us, the difference of organization between a Macrourous and a Bra- 

 chyourous Decapod is much greater than either analogy or anatomy would 

 have led us to suspect. 



The eighth and last section of Dr. Rathke's Work contains his deduc- 

 tions from the previous details, both with reference to the structure of the 

 Crawfish itself, and to its developement as compared with that of other 

 animals. These observations are distributed under the following heads; 

 1, a comparison betweed the trophiand legs of the Crawfish: 2, a com- 

 parison between the structure and developement of the Crawfish, and 

 that of certain nearly related animals: 3, a comparison between the 

 progressive structure of the Crawfish, and the permanent structure of 

 other Crustacea: and 4, a comparison between the structure and deve- 

 lopement of Fertehrata on the one hand, and of the Crawfish on the other. 



With respect to the first point he regards his observations as furnishing 

 a striking confirmation of M. Savigny's hypothesis (now universally ad- 

 mitted) that the maxillae and mandibles of Crustacea are analogous to the 

 legs, or more properly that the one set of organs are merely modifications 

 of the other. The earlier they are examined the more complete is their 

 similarity, both in form and origin ; and it is only after a certain period of 

 their growth that this similarity is lost by the inverse developement of the 

 parts, the basal half of the maxillee increasing in proportion to the termi- 

 nal half of the legs, and vice versa. 



Under the second head the authour compares the results of his own 

 observations with the few similar investigations that have been attempted 



