102 THE FROG. 



and the formation of the segmentation nucleus, this latter loses 

 its spherical form and becomes spindle-shaped, the yolk granules 

 at the same time showing a tendency to arrange themselves 

 along lines radiating outwards from the ends of the spindle. 



The nucleus now divides into two halves, which move away 

 from each other ; the yolk granules tend to aggregate themselves 

 around the two nuclei, and a thin vertical plate of finely granular 

 protoplasm, almost free from yolk granules, is left, dividing the 

 egg into two halves. This plate, which soon becomes pigmented, 

 splits vertically into two, the split appearing near the centre of 

 the egg, and at first not reaching to its surface. 



At the upper or black pole of the egg a depression now ap- 

 pears, at first as a small pit, and then elongating to form a groove, 

 which rapidly extends all round the egg. The groove deepens, 

 and, meeting with the split already present in the interior of the 

 egg (Fig. 46), divides the whole egg into two completely separate 

 and equal parts, the plane of division corresponding with the 

 vertical pigmented plane mentioned above. 



This first plane of division is stated to correspond to the 

 median sagittal plane of the future embryo and adult ; i.e. the 

 two cells into which the egg is divided by the first segmentation 

 plane are said to correspond respectively to the right and left 

 halves of the body of the frog. 



Each of the two nuclei soon becomes spindle-shaped, and 

 then divides into two ; and a second cleft is then formed in a 

 similar manner to the first. This second cleft is also a vertical 

 one, but in a plane at right angles to the first one ; on its com- 

 pletion the egg is divided into four similar and equal cells, or 

 blastomeres. 



The third cleft is horizontal, but not equatorial, lying (Fig. 

 47) much nearer the upper than the lower pole. It divides each 

 of the four cells or blastomeres into two, an upper smaller and a 

 lower larger one. 



From this stage segmentation proceeds rapidly, but according 

 to no definite rule, the several cells dividing independently of 

 one another. Throughout the process the upper cells divide 

 more rapidly, and are consequently always of smaller size 

 than the lower cells, the latter being hampered by the large 

 number of yolk-granules they contain : in all cases division of the 

 cells is preceded by division of their nuclei, as in the earlier stages. 



