iMAKINK FOOD FISHES. 33 



uumerous, and tlie disk rises aud assumes a couvex ibriu ou the side pointing towards tin; 

 yolk. After segmentation is completed, the disk sinks down and assumes a concave ibrm, 

 aud is then termed the segmentation cavity, or embryonic disk or sack. 



On the seventh day, the first indication of the embryo is defined in a thickened rim 

 of the blastoderm, ou the right-hand portion of the embryonic disk. On the eighth day 

 the neural plate becomes visible, appearing like a cord lying across the egg, when viewed 

 from below, and the embryonic sack has become more elongated. 



On the ninth day the head of the embryo is defined and appears like a thickening 

 mass on the lower part of the ovum ; aud the cerebral, with indications of the optic, 

 vesicles, can also be seen forming at the sides of the head. 



On the eleventh day the first segment of the muscular system has appeared on each 

 side of the neural canal, and the dorsal cord (notochord) can be seen below it. On the 

 twelfth day the optic vesicles are seen more defined, while the muscular system and the 

 notochord are more distinct. 



When the embryo is fourteen days old the rudiments of the heart, pectoral fold, 

 auditory canal and intestines can be seen. The stellated pigment cells are now also 

 making their appearance. 



On the fifteenth day the heart appears like a spherical cavity surrounded with a wall 

 of connected cells ; the fore-brain is further developed, aud the optic system more advanced. 

 The primitive lateral fin-folds cau now be observed. The embryo has grown considerably 

 aud is lying near the membrane of the egg in the shape of a half moou. 



On the sixteenth day the tail of the embryo cod becomes free and is twisted over to 

 one side ; the vent is also defined. 



On the eighteenth day the heart is so well developed that it is seen to pulsate 

 regularly, and the eyes are beginuing to receive their colour. The pigment cells are also 

 increasing in number, and the tail is now so well developed that it begins to straighten 

 and move. 



In water having a temperature of 40 degrees Fahr. the embryo cod will hatch, or 

 break through the egg, in twenty or twenty-one days after impregnation. It escapes by 

 bursting its enveloping membrane with vigorous strokes of the tail. For a day or two 

 previous, the imprisoned embryo has been showing by its constant movements that it was 

 eager to enter on its new aud freer life in the great world of waters. The tail is the 

 weapon by which it effects its freedom aud is rrsually pushed out first through the 

 gaping opening. The body, with a yolk-sack hanging to its under side follows. The 

 mouth and digestive oi"gans are not yet formed ; but the yolk-sack, which has been 

 formed from the egg, and is large in proportion to the fish, contains deutoplasm 

 an albuminous substance, which furnishes its nutriment for the first ten or twelve 

 days, until the mouth and digestive organs are developed and the young fish can seek 

 food for itself 



The baby cod, thus liberated, at once makes attempts to swim ; but its tail, for the 

 first day after birth, remains covered, and in consequence its movements are circular, and 

 it spins rovind in spiral fashion. Soon, however, the tail straightens and it is able to 

 swim right ahead, aud is seen gamboling through the water, evidently in a state of enjoy- 

 ment. Its sack of food, however, greatly interferes with its movements, and it is not till 

 this is absorbed that it can swim swiftly. During this period the cod nurslings are kept 



Sec. IV., 1892. 5. 



