6Uo REPORT OF Cr^MMlbSlONKi; OF FISH AN]> FISHERIES. [144' 



EXPLANATION OF PLATE XII. 



FlO. 49. Head of embryo cod on the twenty-seventh day after inii)iognation 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 sui)ported 

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

 the curved cartilaginous rods which sujjport the gills g. The mouth 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 traver.se 

 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 upward.s, and now forms the 

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

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

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

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

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

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

 distinguished, the pronephros. The allantois al has undergone liitle 

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

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

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

 icle an has undergone further dcvolopmont, inasmuch as tlu' semicircu- 

 lar canals are defined, the ingrowing ridges from the walls of the ])rinj- 

 itive vesicle having met and divided the cavity into the rudimentary 

 ampulla; and canals, while the otoliths asterisk as and sagitta .s are seen 

 iu place. The dermal sensory elevations «/i are seen to have filamontouu 

 prolongations liaekwards and forwards which will probably connect them 

 into a continuous lateral series, the rudiment of the lateral liiie.sy.stem of 

 the adult. The complicated stellate ])igment colls pi have aggregated 

 together iu delinile ti'acts on the tail and over the endothelium (peri- 

 toneum) of the visceral cavity, x 85. 



