' xii PRACTICAL DIRECTIONS 213 



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 ectoderm, 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-Jin ; 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, 1-2, 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. i, 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, 5-8.) 



Dissection of tadpole. 



Pin down a full-sized tadpole under water in a small dissecting dish, 

 with the ventral surface uppermost, inserting small pins through the tail 

 only. Carefully dissect off the ventral body walls, and note the coiled 

 intestine, the heart, the internal gills, &c. (Fig. 66, D, D 1 , E. ) 

 Sketch. 



It is rather a difficult task, and requires much time, to prepare sections 

 of the early stages of the frog-embryo in order to make out the formation 

 of the ectoderm, endoderm, and mesoderm, the relations of the seg- 



