CRUSTACEA. 



427 



cells in Palsemon do not, at first, form a continuous layer enclosing 

 a central cavity, while they do so in Astacns. . 



B 



Vff 



Jtd 



FIG. 239. Two STAGES IN THE DEVELOPMENT OF PAI^MON SEEN IN SECTION. 



(After Bobretzky.) 



A. Gastrula stage. 



B. Longitudinal section through a late stage. Inj. hypoblast; sg. supra-cesopha- 

 geal ganglion; vg. ventral nerve cord; hd. proctodasum ; st. stomodaeum. 



The mesoblast appears to be formed of cells budded off from the 

 anterior wall of the archenteron (Astacus, fig. 237 D), or from its 

 lateral walls generally (Palsemon). They make their first appearance 

 soon after the invaginatiou of the hypoblast has commenced. The 

 mesoblast cells are at first spherical, and gradually spread, especially 

 in an anterior direction, from their point of origin. 



According to Reichenbach there are formed in Astacus at the Nauplius 

 stage a number of peculiar cells which lie speaks of as 'secondary mesoblast 

 cells.' His account is not very clear or satisfactory, but it appears that 

 they originate (fig. 237 F) in the hypoblast cells by a kind of endogenous 

 growth, and though they have at first certain peculiar characters they soon 

 become indistinguishable from the remaining mesoblast cells. 



Towards the end of the Nauplius period the secondary mesoblast cells 

 aggregate themselves into a rod close to the epiblast in the median ventral 

 line, and even bifurcate round the mouth and extend forwards to the 

 extremity of the procephalic lobes. This rod of cells very soon vanishes, 

 and the secondary mesoblast cells become indistinguishable from the 

 primary. Reichenbach believes, on not very clear evidence, that these 

 cells have to do with the formation of the blood. 



General form of the body. The ventral thickening of epiblast or 

 ventral plate, continuous with the invaginated patch already men- 

 tioned, forms the first indication of the embryo. It is at first oval, 

 but soon becomes elongated and extended anteriorly into two lateral 

 lobes the procephalic lobes. Its bilateral symmetry is further in- 

 dicated by a median longitudinal furrow. The posterior end of the 

 ventral plate next becomes raised into a distinct lobe the abdomen 

 which in Astacus at first lies in front of the still open blastopore. 



