438 



DEVELOPMENT OF ORGANS. 



The primitive mesenteron in Palaemoii differentiates itself into the 

 permanent mid-gut and liver in a manner generally similar to that in 

 Astacus, though the process is considerably less complicated. A distinct 

 layer of cells separates itself from the outer part of the yolk pyramids, 

 and gives rise to the glandular lining both of the mid-gut and of the liver. 

 The differentiation, of this layer commences behind, and the mid-gut very 

 soon communicates freely with the proctodteum. The lateral parts of 

 the primitive rnesenteroii become constricted into four wings, two directed 

 forwards and two backwards ; these, after the yolk in them has become 

 absorbed, constitute the liver. The median part simply becomes the me- 

 seiiterou. The stomachic end of the stomoda^um lies in contact with the 

 mesenteron close to the point where it is continued into the hepatic 

 diverticula, and, though the partition-wall between the two becomes early 

 very thin, a free communication is not established till the yolk has been 

 completely absorbed. 



The alimentary tract in the Isopoda is mainly if not entirely formed 

 from the proctodaeurn and stomodaBum, both of which arise before any other 

 part of the alimentary system as epiblastic invagiuations, and gradually 

 grow inwards (tig. 244). In Oniscus the liver is formed as two discs 

 at the surface of the yolk on each side of the anterior part of the body. 

 Their walls are composed of cubical cells derived from the yolk cells, the 



pr 



FlG. 2i. TWO LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF ONISCUS MURARIUS. 



(After Bobretzky. ) 



i>t. stornodffium; pr. proctodasum; hy. hypoblast formed of large nucleated cells 

 imbedded iu yolk; w.mesoblast; vg. ventral nerve cord; sy. supra-cesophageal ganglion; 

 //. liver; do. dorsal organ; zp. rudiment of masticatory apparatus. 



origin of which was spoken of on p. 429. These two discs gradually take 

 the form of sacks (tig. 244 B, li.) freely open on their inner side to the 

 yolk. As these sacks continue to grow the stomodseum and proctodseum 

 do not remain passive. The stomodseum, which gives rise to the oesophagus 

 and stomach of the adult, soon exhibits a posterior dilatation destined to 

 become the stomach, on the dorsal wall of which a well-marked prominence 

 the earliest trace of the future armature is soon formed (fig. 244 B, zp). 

 The proctodseum (pr) grows with much greater rapidity than the stomo- 

 daeiim, and its end adjoining the yolk becomes extremely thin or even broken 

 through. In the earliest stages it was surrounded by the yolk cells, but 

 in its later growth the yolk cells become gradually reduced in number 

 and appear to recede before it so much so that one is led to conclude 



