THE DEVELOPMENT OF THE LIMBS. 217 



depressions indicate the boundaries of the posterior 

 cephalic and the thoracic somites ; and pairs of eleva- 

 tions, similar to the rudiments of the antennules and 

 antennae, appear upon them in regular order from before 

 backwards (fig. 59, C). 



In the meanwhile, the extremity of the abdomen 

 flattens out and takes on the form of an oval plate, 

 the middle of the posterior margin of which is slightly 

 truncated or notched ; while, finally, transverse constric- 

 tions mark off six segments, the somites of the abdomen, 

 in front ot this. Along with these changes, four pairs 

 of tubercles grow out from the sternal faces of the four 

 middle abdominal somites, and constitute the rudiments 

 of the four middle pairs of abdominal appendages. The 

 first abdominal somite exhibits only two hardly percept- 

 ible elevations in place of the appendages of the others, 

 while the sixth seems, at first, to have none. The ap- 

 pendages of the sixth somite, however, are already formed, 

 though, singularly enough, they lie beneath the cuticle 

 of the telson and are set free only after the first 

 ecdysis. 



The rostrum grows out between the procephalic lobes ; 

 it remains relatively very short up to the time that the 

 young crayfish quits the egg, and is directed more down- 

 wards than forwards. The lateral portions of the cara- 

 pacial ridge, becoming deeper, are converted into the 

 branchiostegites, and the cavities which they overarch 

 are the branchial chambers. The transverse portion of 



