172 BULLETIN OF THE UNITED STATES FISH COMMISSION 



EXPLANATION^ OF PLATE IV. 



Fig. 16. — Spanish mackerel four days after development began and three days after 

 it had left the egg. The chondrocranium is much more developed than in 

 Fig. 15, as is apparent from the development of the mandibular, hyoid, 

 and branchial arches, as well as the trabecuhe cranii, parachordals, and 

 the investment of inciiiient cartilage covering the ear capsules. In con- 

 sequence of the advanced development of the mandibular and hyoid arches 

 the mouth is now wide open. The caraco-scapular element esc at the base 

 of the breast fin is a slender hyaline rod, different in its histological struct- 

 ure from that of the cartilage of the head bones, and marks the point 

 where a transverse fold of the hypoblast partially closes off the pericardiac 

 cavity from that of the abdomen. The liver Iv is now more fully developed 

 and is already divided into follicles. There is one turn of the intestine 

 upon itself, which in this instance contains the remains of food at. its 

 hinder extremity. The circulation is now fully established, the dorsal 

 aorta extending back for about half the length of the animal; at its pos- 

 terior extremity, cv, it becomes the caudal vein which carries the blood 

 forward to the heart, but not until a considerable part of it passes through 

 a system of vessels which traverse the viscera externally, when it is poured 

 directly into the venous sinus at, and so into the general circulation 

 through the heart and gills. X 50. 



Fig. 19. — Head of young Spanish mackerel on the sixth day after hatching, partly 

 diagrammatic, showing the air-bladder ab, the urinary bladder al, the 

 liver Ir, more developed than in Fig. 16. The oil sphere os has been nearly 

 absorbed. The chondrocranium is very much more fully developed, while 

 well defined conical teeth have made their appearance on the papillae of 

 the dermal epithelium of the lower jaw, which is now longer than the upper. 

 The opercular fold is also more prominent. X 40. 



