xii PRACTICAL DIRECTIONS 213 



II. Early stages in segmentation, during which the 

 individual cells can be made out by means of a lens. (Fig 6^ 

 A F.) 



III. Later stages in segmentation, during which the 

 individual cells cannot be distinguished without the aid of 

 a microscope. Note the gradual enclosure of the white by 

 the black hemisphere, until only a small rounded yolk-plug 

 is left, filling in an aperture the blastopore in the black 

 layer or ectoderm, as it is now called. The yolk-plug is 

 continuous with a mass of yolk-cells, enclosed by the ecto- 

 derm, from which the endoderm and mesoderm are developed. 

 (Fig. 64, G I.) 



IV. The gradual flattening and elongation of the embryo, 

 and the position of the blastopore at the posterior end of the 

 dorsal region ; the formation of the medullary folds and 

 groove along the dorsal side, and the closure of the blastopore. 

 (Fig. 64, H-J.) 



V. The closure of the medullary groove, the formation of 

 the head with its suckers and of the tail with its tail-fin ; as 

 well as the appearance of the eyes, ear-sacs, branchial 

 arches, external gills, and the involutions for the mouth and 

 anus, which, however, do not open until after the tadpole 

 is hatched. (Fig. 64, K L.) 



After hatching. 



VI. The appearance of the horny jaws and papilla, and 

 the further development of the head, external gills, and tail. 

 (Fig. i, i2, and Fig. 66, A, B.) 



VII. The formation of the operculum, the closure of the 

 right opercular aperture, and the disappearance of the 

 external gills. The bud-like rudiments of the hind-limbs, and 

 their further development. The fore-limbs remain for a 

 long time hidden beneath the operculum. (Fig. i, 3 4, 

 and Fig. 66, C E.) 



Metamorphosis. 



VIII. The gradual change in the form and colour of the 

 head and body ; the widening of the mouth and loss of 

 the horny jaws and suckers ; the appearance of the fore- 

 limbs from beneath the operculum, and the shrinking of the 

 tail. (Fig. i, 58.) 



Dissection of tadpole. 



Pin down a full-sized tadpole under water in a small dis- 

 secting -dish, with the ventral surface uppermost, inserting 

 small pins through the tail only. Carefully dissect off the 



