584 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [128] 



EXPLANATION OF PLATE IV. 



Fio. 19. C'lil's egg on the eighth day, showing the appearance of the neural or medul- 

 lary plate p. st, the contour of the segmentation cavity sg, and the blas- 

 todermic rim r continuous with the embryo, viewed from below, x 57. 



Fig. 20. Cud's egg on the ninth day after impregnation, showing the embryo from 

 the side, the head being already defined as a well-marked thickening to 

 the right of A, with indications of the optic vesicles, x 30. 



Fig. 21 . Embryo of the same age, viewed obliquely, with the head directed forwards ; 

 the cerebral structures forming, with indications of the beginnings of 

 the optic vesicles at the sides of the head at op. x 30. 



Fig. 22. Embryo ten days old ; the blastoderm has grown very considerably, and all 

 of the yelk which now remains exposed is that above the blastodermic 

 rim r ; the lumen of the segmentation cavity is seen to extend from the 

 head at A to the lower edge of the blastodermic rim at the left side. 

 The optic vesicles are defined, and the first cerebral vesicle has been 

 formed and become very much flattened laterally or compressed, so that 

 its "keel " dips down into the yelk very considerably. 



IIg. 23. Embryo somewhat older than the preceding (the body only being shown) ; 

 the first pair of muscular segments pv have appeared on either side of 

 the neural or medullary canal and the chorda dorsalis cli below it. 



Fig. 23 a. The head of the same from directly in front, showing the flattened cere- 

 bral vesicles and optic vesicles in section. 



