464 



MERISTIC VARIATION. 



[part I. 



149, I. The first furrow, 1, 1, divides the blastoderm into two 

 halves and corresponds with the future longitudinal middle line. 

 The second furrow, 2, 2, is at right angles to this, dividing the 

 blastoderm into anterior and posterior halves, and the third 

 furrow, 3, 3, passes as shewn in Fig. 149, V. 



In the subsequent segmentations various irregularities were 

 seen in single eggs, some of the variations being bilaterally sym- 

 metrical while others were confined to a particular half or to a 

 particular quadrant. For example, in some ova the nuclei of the 

 cells formed from the left half of the blastoderm, excepting those 

 next the median axis posteriorly (Figs. 149, II and 150, I), began 

 to divide before those of the right side and reached an advanced 

 stage of karyokinesis while the nuclei of the right half were still 

 resting. The nuclei of each half kept time very nearly (for details 

 see original figures). This curious variation was seen in three 

 (perhaps four) ova all taken from one mother. 



In another the nuclei of the two anterior quadrants al, ar, in 

 their divisions kept ahead of those of the posterior quadrants. 

 Fig. 149, IV. represents an ovum in which the nuclei of the right 

 posterior quadrant on the contrary divided before those of the 3 

 other quadrants. 



Another variation is shewn in Figs. 149, VI and 150, II. There 

 the four blastomeres shaded had either been never fully divided 

 from each other or had subsequently fused together symmetrically 

 on each side. 



/ IT 



Fig. 150. Variations in segmentation of ovum of Loligo pealei. I. Case in 

 which the nuclei of cells of the left half of the blastoderm began to divide pre- 

 cociously. II. Case in which the blastomeres of the areas ar' and al' were not 

 distinct from each other. (After Watase.) 



Fig. 149, V, illustrates another remarkable Meristic variation 

 which symmetrically affected the portions shaded. In both of 

 these shaded segments the nuclei divided into three by triple 

 karyokinesis, forming " triasters." 



