No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 131 



dently has not carried his observations far enough back to 

 observe the scattering forward of the mesoderm from the blas- 

 topore, and finding mesoderm cells below the lateral ventral 

 bands has imagined that they have split off from them. It 

 remains to be seen whether further observations on the Bra- 

 chyura will confirm Lebedinski's statements ; it may be noted, 

 however, that the recent work of Cano ('93) on Maja does not 

 afford any support to them. 



In this connection mention may be made of Weldon's ('92) 

 observations on Crangon, in which he finds a patch of mesoderm 

 cells on either side of what he takes to be the blastopore and 

 beneath apparently the posterior ends of the lateral embryonic 

 bands. I have observed the same arrangement in Palaemonetes 

 and VirbiiLS but cannot agree with the interpretation Weldon 

 puts upon it. He regards what he has found as a partial con- 

 firmation of Nusbaum's views, but in reality the conditions are 

 quite different, since Nusbaum derives the mesoderm from the 

 ectoderm of the ventral surface of the naupliar region, while, 

 even granting that Weldon's views are correct, it is formed 

 from the meta-naupliar region in Crangon, in my opinion a 

 very important difference. But, in addition, Weldon identifies 

 what he terms the ventral neuro-muscular plates of his Fig. 6 

 with the thoracico-abdominal plates of Astaciis, and this is 

 where I believe he has fallen into error. As I interpret the 

 similar appearance in Virbuis these neuro-muscular plates are 

 in reality part of the blastopore which is elongated laterally, 

 and they are entirely composed of mesoderm cells, not yet 

 being covered in by ectoderm, a view which, it seems to me, is 

 corroborated by the appearance which is seen in section and 

 which Weldon represents in his Fig. 16. His Fig. 6 is not at 

 all comparable, as he supposes, to Reichenbach's Fig. 3, but is 

 of a much earlier stage. Why the mesoderm should arise as 

 two lateral masses rather than as a single mass situated immedi- 

 ately in front of the vitellophag-endoderm mass, may perhaps 

 be explained by the separation of the two lateral ventral bands 

 in early stages. 



The remaining observations which belong to the second 

 group of results require but little discussion. The views of 



