688 KEPORT OF COMMISSIONER OF FISH AND FISHERIES. [lo'i] 



EXPLANATION OF PLATE VI. 



Fig. 29. Head of embryo cod seventeen days old, viewed from below, showing the 

 cerebral vesicles differentiated, and the hypoblastic and mesodermal 

 tracts of cells i, from which the trabeculse cranii, etc., are developed. The 

 anterior dilated extremity of the embryonic heart is bending to the left, 

 and is destined to become the auricle and venous sinus, x 55. 



I'iGS. 29 a and 29 b represent the caudal ends of embryos thirteen days old, and relate 

 to the history of the blastopore and Kupffer's vesicle, x 55. 



Fig. 30. Head of embryo cod fifteen days old, showing the disposition of the tissues in 

 the head region, indicated by the large rounded cells. The heart A, aa 

 compared with Fig, 29, is a mere spherical sinus or cavity, with a wal 

 of mesodermal cells, x 65. 



Figs. 30 a and 30 i relate to the history of the blastopore, x 55. 



