CRUSTACEA. 433 



After being enclosed by the protoplasmic cells the yolk divides, and 

 gives rise to a number of cells, which probably supply the material for the 

 walls of the mesenteron. The external layer of protoplasm forms the 

 so-called blastoderm, and soon (Arnold, Lang) becomes thickened on the 

 dorsal surface. 



The embryo is next divided by two constrictions into three segments ; 

 and there are formed the three appendages corresponding to these, which 

 are at first simple. The two posterior soon become biramous. The larva 

 leaves the egg before any further appendages become formed. 



Comparative development of the organs. 



Central nervous system. The ventral nerve cord of the Crustacea 

 develops as a thickening of the epiblast along the median ventral 

 line; the differentiation of which commences in front, and thence 

 extends backwards. The ventral cord is at first unsegmented. The 

 supra-cesophageal ganglia originate as thickenings of the epiblast of 

 the procephalic lobes. 



The details of the above processes are still in most cases very imper- 

 fectly known. The fullest account we have is that of Reichenbach (No. 488) 

 for Astacus. He finds that the supra-oesophageal ganglia and ventral cord 

 arise as a continuous formation, and not independently as would seem to be 

 the case in Chsetopoda. The supra-cesophageal ganglia are formed from the 

 procephalic lobes. The first trace of them is visible in the form of a pair 

 of pits, one 011 each side of the middle line. These pits become in the 

 Nauplius stage very deep, and their walls are then continued into two 

 ridges where the epiblast is several cells deep, which pass back-wards one 

 on each side of the mouth. The walls of the pits are believed by Reichen- 

 bach to give rise to the optic portions of the supra-cesophageal ganglia, 

 and the epiblastic ridges to the remainder of the ganglia and to the 

 circum-cesophageal commissures. At a much later stage, when the ambu- 

 latory feet have become formed, a median involution of epiblast in front 

 of the mouth and between the two epiblast ridges gives rise to a central 

 part of the supra-cesophageal ganglia. Five elements are thus believed by 

 Reichenbach to be concerned in the formation of these ganglia, viz. two 

 epiblast pits, two epiblast ridges, and an involution of epiblast between 

 the latter. It should be noted however that the fate neither of the pair 

 of pits, nor of the median involution, appears to have been satisfactorily 

 worked out. The two epiblast ridges, which pass back from the supra- 

 cesophageal ganglia on each side of the mouth, are continued as a pair 

 of thickenings of the epiblast along the sides of a median ventral groove. 

 This groove is deep in front and shallows out posteriorly. The thickenings 

 on the sides of this groove no doubt give rise to the lateral halves of the 

 ventral cord, and the cells of the groove itself are believed by Reichenbach, 

 but it appears to me without sufficient evidence, to become invaginated 

 also and to assist in forming the ventral cord. When the ventral cord 

 becomes separated from the epiblast the two halves of it are united in the 

 middle line, but it is markedly bilobed in section. 



In the Isopoda it would appear both from Bobretzky's and Bullar's 

 observations that the ventral nerve cord arises as an unpaired thickening 

 of the epiblast in which there is no trace of anything like a median in- 

 volution. After this thickening has become separated from the epiblast 



B. E. 28 



