366 M. Rathke on the Development of' the Decapodes. 



Slight, or only moderately great deviations in the proportions of the sepa- 

 rate external organs, like those (excluding the eyes) occurring in the newly 

 born Decapodes, can afford no grounds for our forming such an opinion regard- 

 ing them as that expressed by Thompson. In perfect specimens of the Asta- 

 cus Leptodactylus, the tentaculse of the male in proportion to the body are at 

 least twice as great as in the female, without our concluding that the female 

 is much less perfect than the male. How completely different, on the other 

 hand, are the relations of many lower Crustacea in their perfect and in their 

 imperfect condition! All the Isopodes, with whose development I am ac- 

 quainted, come into the world with fewer bones than they exhibit in their 

 state of maturity the Bopyrus squillarum has three pair less ; the Cyclopes 

 have no bones when they come out of the egg, and some parts of the dentary ap- 

 paratus are also awanting; the Lepas and Balani resemble their parents just 

 as little as the Cyclopes do when they come out of the egg. These are animals 

 which we can say with justice and reason enter the world in an extremely im- 

 perfect condition ; but, as to the Decapodes, so far as I have examined their 

 development, I must deny such an assertion, and of them I can say nothing 

 less than at the end of their existence in the egg they have exactly the same 

 aspect, and are as fully developed, as the full-grown individuals. No phy- 

 siologist would make a similar assertion regarding a newly born bird or quad- 

 ruped. 



We certainly remark a considerable difference between the animal when it 

 leaves the egg and an old individual, in reference to the form of the whole 

 body ; but this arises from the circumstance of the young one carrying away 

 a considerable quantity of yolk from the egg. The yolk fills up a large, per- 

 haps the largest, portion of the cavities of the body. Hence the greater 

 breadth of the thorax in the matured embryo of the Palaemon and the Cran- 

 gon ; hence the greater thickness of that part of the body in the matured em- 

 bryo, not merely of the Crustacea, but also of the Eriphia ; hence also the 

 circumstance, that in all of them the middle portion of the shield, in propor- 

 tion to the lateral portions, is very much larger, and relatively also much 

 thinner, than afterwards when the creature attains its full maturity. But 

 birds also take with them from the egg a portion of the yolk, and many of 

 them after creeping out have still a very large belly, yet nevertheless no one 

 would assert that birds come into the world in a very imperfect condition. 



The stomach and the liver may perhaps be but little developed when the crab 

 leaves the egg, and the organs of generation may perhaps be entirely awarit- 

 ing ; but I cannot believe that Thompson founded his assertion on these or- 

 gans ; for I have seen specimens of five or six species of crabs which carried 

 eggs ; but none of the eggs, so far as I remember, were larger than poppy 

 seeds. In newly born young ones of these animals, it would, therefore, have 

 been a difficult task to investigate the relations of the internal organs men- 

 tioned above, and which were partly covered with yolk. 



In conclusion, I must remark, that I havo not been able to procure a sight 

 of the original paper by Thompson ; and that I have only seen the abstract 

 of its more important contents which was published in the Isis. Perhaps, 

 therefore, much of what I have stated in this letter is not applicable to the 

 memoir in question ; and it may perchance turn out, that, like Don Quixote, 

 I have been fighting against a windmill. 



