598 KEPOKT OF COMMISSIONER OF FISH AND FISHERIES. [142] 



EXPLANATION OF PLATE XI. 



FlO. 45. Yonng codfish thirty days after impregnation and ten days after it has left 

 the ogg, showing the high dorsal natatory membrane or median fin-fold, 

 of theforward part of the animal reminding one of the form of the dolphin 

 of ancient sculptures. We note but little difference as compared with 

 the younger stage, shown in Fig. 49, except that the blood system has been 

 more fully developed, the dorsal aorta turning upon itself at p on the 

 underside of the tail, to become the caudal vein, which splits up into car- 

 dinal branches anteriorly, which give ofi" vessels which pass over the vis- 

 cera in front of the allantois to finally empty into the venous sinus. At no 

 time has there been a true omphalomeseraic system developed, since the 

 heart does not even j»ulsate until some time after the tail begins to form. 

 x25. 



Fig. 46. Embryo cod twenty-three days after impregnation, from below, showing the 

 pectoral folds ^/, the oesophagus ce, the heart, and the rudimentary gill 

 clefts g. The mouth m, the maxillary hyoidean, and gill arches are indi- 

 cated, while the pericai'diac septum pc, intervening between the heart 

 space and body cavity, is also shown* x 40. 



PlO. 47. Four days' blastoderm of cod's egg in optic section, to show the extent of the 

 segmentation cavity, x 32. 



PlO. 48. A similar disk four and a half days old. x 32. 



