MACKEREL INVESTIGATIONS IN 1897. 13 



At 88 hours after the beginning of development the embryo measures 

 2.3 mm. in length, and the tail has grown out free of the yolk to a 

 length of 0.2 mm. The head has become broad and conspicuous, and 

 measures 0.4 mm. across the eyes. The ear-sacs are still thick-walled, 

 simple vesicles, measuring 0.1 mm. in length. The branchial sense 

 organs are conspicuous. The pigment is now arranged principally in 

 two lines along the margins of the body, a very few scattered cells 

 appearing on its dorsal surface. Yellow pigment makes its first appear- 

 ance behind the forming eyes. 



At 102 hours the embryo measures 2.4 mm., the free tail 0.3 mm. The 

 median fin ridge has begun to develop on the tail, which has now been 

 deflected to the left side of the embryonic axis. The alimentary canal 

 (mesenteron) is still an open groove for most, if not all, of its length. 

 The mouth depression is appearing, with slightly marked maxillary 

 and mandibular processes inclosing it. The branchial clefts appear as 

 faintly defined furrows. The head is still very broad and flat. The 

 heart pulsates actively, but somewhat spasmodically. The oil-sphere 

 is closely applied against the base of the tail and is undergoing rapid 

 absorption. Its protoplasmic sheath has fused with the blastoderm, and 

 the pigment cells, which have now developed on its internal surface, 

 have united by anastomosing j>rocesses to constitute a reticulum. 



At 114 hours the embryo is 2.6 mm. in entire length ; the free tail is 0.3 

 mm. The caudal fin-fold is quite prominent. The tail has now turned 

 with its flat side toward the yolk. There are about 73 myotomes. The 

 head has not yet begun to rise freely from the yolk-sac. A thickening 

 near the anterior end of the alimentary canal indicates the outgrowth 

 of the liver. The alimentary groove seems to be entirely closed. 



Hatching usually takes place at 120 hours after impregnation. The 

 newly hatched larva measures 3.3 mm. in length: the yolk-sac 1.3 mm. 

 in length and 0.8 mm. in depth. The oil-drop has become much reduced 

 and its inner surface flattened. It measures 0.28 by 0.15 mm. The 

 head projects slightly beyond the yolk-sac. The mouth and gill slits 

 are not yet opened, but there is a peculiar cleft beneath the edge of 

 the yolk-sac, just over the branchial region. The cephalic sinus is 

 small. Pigment is just beginning to appear in the retina. The yolk- 

 sac is ovoid, and deepest toward its posterior end. The oil-drop is 

 embedded in the posterior ventral part of the yolk-sac, having been 

 pushed forward a short distance from the base of the tail, and at the 

 same time has rotated so that the flattened pigmented surface is now 

 outermost. Considerable individual variation was observed in the 

 exact position of the oil-drop. There is no black pigment on the sur- 

 face of the yolk-sac, of the median fin-fold, or on the ventral surface of 

 the embryo, except where a few chromatophores sometimes migrate 

 downward over the base of the tail. On the head the black pigment 

 has now formed a conspicuous reticulum extending to the extremity of 

 the snout. On the body and tail the pigment is more scattered, but is 

 chiefly confined to two series of chromatophores located along the base 

 of the dorsal fin and the lateral aspects of the tail. The individual 



