130 



THE BOOK OF NATURE STUDY 



being cut with an invisible knife. It divides into a right and a 

 left half, then into an anterior and a posterior half, then into a 

 dorsal and a ventral half. By three cleavages at right angles 

 to one another the first two vertical and the third equatorial 

 it divides into eight cells, the four in the upper hemisphere being 

 somewhat smaller than the four in the lower hemisphere. Quickly 

 it forms a ball of many cells, though it is not any larger than it 

 was, for growth has not begun. The cells go on multiplying, and 

 the embryo becomes a little larger. Gradually the important 

 organs begin to make their appearance, the dorsal nerve cord 

 and the brain ; the food canal ; the dorsal supporting rod or 

 notochord ; the foundations of the rest of the skeleton ; the 

 beginnings of musculature, and so on. The embryo moves within 

 its envelope of jelly, and wriggles itself free. It is " hatched." 



The Newly Hatched Embryo. The head is clearly distinct 

 from the body; a dimple indicates the future position of the 



mouth ; a pit indicates the nostrils ; a 

 grooved crescent behind the mouth depres- 

 sion produces a gum-like slime which 

 attaches the helpless embryo to water- 

 weed ; the tail is rudimentary ; the eyes 

 have not yet reached the surface (the 

 FIG. 41. The newly hatched e ye of a backbone animal always begins 



tadpole, showing the horse- , -i x J.-L -L \ 



shoe-shaped cement gland as an outgrowth from the brain) ; a minute 



bud on each side of the head is the rudi- 

 ment of the external gills, and vertical 

 grooves beside it hint at the future gill- 

 clefts. The newly hatched embryo may 

 be described as mouthless, tailless, limbless, 

 gill-less, and eyeless. 



Early Changes after Hatching. The tail 

 grows, and with a lens there may be seen 



a transparent tail-fin and wedge-like muscle segments. Three 

 external gills bud out, the first large, the other two small. The 

 mouth is opened and has fleshy lips ; the other end of the food 

 canal opens ; the nostril-pits open into the mouth ; the gill-clefts 

 open; and the eyes become faintly visible. Hitherto the larva 

 has been growing at the expense of its legacy of yolk ; it now 



(AG), the external gills 

 (EG) slightly developed, 

 the position of the eye (E), 

 which is not yet exposed, 

 and the position of the ear. 

 The bulging on the ven- 

 tral surface indicates the 

 mass of food-yolk within 

 the body. 



