224 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



on the one hand, and the cephalic integument on the 

 other. 



The mesoblastic cells give rise to the layer of con- 

 nective tissue which forms the deeper portion of the 

 integument, and to that which invests the alimentary 

 canal ; to all the muscles ; and to the heart, the vessels, 

 and the corpuscles of the blood. The heart appears 

 very early as a solid mass of mesoblastic cells in the 

 tergal region of the thorax, just in front of the origin 

 of the abdomen (figs. 57, 58, 59, h). It soon be- 

 comes hollow, and its walls exhibit rhythmical con- 

 tractions. 



The branchiae are, at first, simple papillae of the integu- 

 ment of the region from which they take their rise. 

 These papillae elongate into stems, which give off lateral 

 filaments. The podobranchiae are at first similar to the 

 arthrobranchiae, but an outgrowth soon takes place near 

 the free end of the stem, and becomes the lamina, while 

 the attached end enlarges into the base. 



The renal organ is stated to arise by a tubular involu- 

 tion of the epiblast, which soon becomes convoluted, and 

 gives rise to the green gland. 



The central nervous system is wholly a product of the 

 epiblast. The cells which lie 'at the sides of the longi- 

 tudinal groove already mentioned (fig. 58, mg), grow in- 

 wards, and give rise to" two cords which are at first 

 separate from one another and continuous with the rest 

 of the epiblast. At the front end of the groove a 



