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 



as an outgrowth from the brain) ; a minute 

 bud on each side of the head is the rudi- 

 (EG) slightly developed, me nt 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, 



?ss, 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 



and the position of the ear. 

 The bulging on the ven- 

 tral surface indicates the 

 mass of food-yolk within 



the body. 



