'600 REPORT OF COxMMISSIONER OF FI8H ANlJ FISHERIES. [144] 



EXPLANATION OF PLATE XII. 



Fig. 49. Head of embrjo cod on tlie twenty-.seventh day after impregnation and one 

 week after hatching, seen from the side. The breast fin has assumed a 

 nearly vertical position. Meckel's cartilage forms the skeletal basis of the 

 lower jaw, and the palatopterygoid cartilage and quadrate constitute the 

 major portion of the sides of the upper jaw. The tongue t is supported 

 by a glosso-hyal, followed by other hyal cartilages, to which are attached 

 the curved cartilaginous rods which support the gills g. The month is 

 now wide open, and the young fish snaps its jaws vigorously. The circu- 

 lation has been established ; a dorsal aorta and caudal vein now traverse 

 the mesoblastic tissue mes below the chorda, and the blood corpuscles 

 have acquired a reddish tinge. The yelk-sack d is being gradually 

 emptied of its contained deutoplasm. The splanchnopleural covering of 

 the forepart of the yelk-sack has been pushed upwards, and now forms the 

 hinder wall of the pericardiac cavity cc. The liver Jv is more distinctly 

 developed than formerly, and occupies a lower position in the visceral 

 cavity above it, and dorsad of the structxire y has increased in size, and 

 appears to represent the dorsad portion of the rudimentary liver, while in 

 front of it lies the aii'-bladder proper, covered with pigment cells. In 

 front of the air-bladder again a convoluted tubular organ j)hjj may be 

 distinguished, the pronephros. The allantois al has undergone little 

 advancement, but the intestine can now be divided into three regions: 

 that of the gullet from w to the liver, the mid-gut region from the liver 

 to the constriction ic, and the hind-gut from ic to v. The auditory ves- 

 icle au has undergone further development, inasmuch as the semicircu- 

 lar canals are defined, the ingrowing ridges from the walls of the prim- 

 itive vesicle having met and divided the cavity into the rudimentary 

 ampullae and canals, while the otoliths asterisk as and sagitta s arc seen 

 in i)lace. The dermal sensory elevations sli are seen to have filamentous 

 prolongations backwards a nd forwards which will i»robably connect them 

 into a coutinnous lateral series, the rudiment of the lateral line system of 

 the adult. The complicated stellate pigment cells pi have aggregated 

 together in defiuite tracts on the tail and over the endothelium (peri- 

 toneum) of the visceral cavity, x 85. 



