io 6 



AN INTRODUCTION TO ZOOLOGY 



to the first starts, and in a similar way cuts the two cells into four. 

 The next furrow is at right angles to both the preceding, and so is 

 equatorial and not longitudinal. It is situated nearer the upper 

 pole, and consequently divides the egg into four smaller and four 

 larger cells. This process of dividing up, termed segmentation, 

 goes on fairly rapidly but less regularly, and as a result there is pro- 

 duced a mass of cells very small at the animal pole and larger at the 

 vegetative pole. Sections show that the ovum is now hollow inside, 

 the cells being arranged around an excentric space known as the 

 segmentation cavity or blastocoel, and the whole structure is termed 

 a blastula. The further details of these changes will be dealt with 

 again later, and only the external alterations noted here. The black 







FIG. 35. -The life-history of a frog. After Brehm. 



1-3, developing ova ; 4, newly hatched forms hanging to water-weed? ; 5, 6, stages 

 with external gills; 7-10, tadpoles during emergence of limbs; u, tadpoles with 

 both pairs of limbs apparent ; 12, metamorphosis to frog. 



cells start to grow down over the others, and at about the end of the 

 fourth day the whole egg is black. At the expiration of a week the 

 embryo, as we may now term it, is distinctly oval, and three or four 

 days later a head, body and tail portions are visible. After a fort- 

 night the young animal breaks from the gelatinous envelope and 

 becomes a tadpole. It is now a free swimming comma-like creature 

 with a horseshoe-shaped structure, the sucker, on the ventral surface 

 of the head, by means of which it adheres to weeds or other objects. 

 The food required for its growth so far has been entirely derived from 

 the yolk stored in the egg to start with, and it is not until a few 

 days after hatching that a mouth with horny beak-like jaws develops, 

 and the animal is capable of feeding for itself. As a result of this 



