2 oo KINGSLE V, [\^ol. V 1 1 1 . 



on the ventral side until a later date, and is then apparently 

 obliterated by a flattening of the cavities and a growing to- 

 gether of their walls. In the fifth somite, however, I can 

 speak with more confidence, — for here the coelomic cavities 

 divide into two moieties, a dorsal and a ventral, and the latter 

 remains upon the ventral surface and gives rise, in a way soon 

 to be described, to the nephridia. 



Concerning the fate of the dorsal portion of the coelom I 

 am uncertain. It persists, as has been said, as a perfectly 

 distinct cavity with epithelial walls on either side of the central 

 circulatory organ in the latest stages which I have studied. 

 From its position and from its posterior termination I am 

 inclined to think that this portion of the coelomic epithelium 

 is finally converted into the reproductive organs. In young 

 Limuli an inch and a half in length I have found no traces of 

 the cavity except as it might be represented in the gonads. 

 I regret also that I have not been successful in tracing the 

 history of the reproductive ducts. There are, however, so 

 many lacunae developed in the genital somite that I have not 

 been able to follow the fate of the lower portion of the coelom 

 in that somite. 



From this point the subsequent history of the mesoderm 

 is best followed under the headings of the different organs, 

 but before taking them up it is well to consider the facts 

 already described. 



Comparisons . — The only previous papers ^ dealing with the 

 mesoderm of Limulus are those by Patten ('90), Kishinouye 

 ('91, '92) and myself ('85), and Patten's remarks are only inci- 

 dental to the discussion of an entirely different question. 

 According to him there is a short slit-like invagination at 

 the posterior end of the embryo, and from the walls of this 

 inpushing much of the entoderm and mesoderm is produced, 

 essentially as described above for mesoderm alone. Again 



1 Possibly an exception should be made made in favor of Dr. Packard's last 

 paper, where ('85, p. 269) a few statements are made concerning this layer. He 

 recognizes (Fig. 3) two coelomic pouches in the region in front of the first pair 

 of appendages; but his figures clearly show that he has not seen the true coelom; 

 the cavities described being either lacunar or artifacts. The coelom does not 

 exist in the plane of his section in the stage he has studied. 



