50 



CLEAVAGE 



II 



course, two such planes, for example in the Ctenophore egg, and 

 the Teleostean egg. In the spiral type, firstly, the successive 

 cleavages are oblique to the egg-axis; secondly, never more 

 than three surfaces of contact intersect in one line, so that 

 cross- or polar-furrows are developed between opposite blasto- 

 meres in for instance the four-celled stage ; while in the third 

 place, successive quartettes of micromeres are thrown off at 



i: i nL P. 



FIG. 10. Three segmentation stages in the blastoderm of Sepia offi- 

 cinalis ; the segmentation is of the bilateral type. I, left ; r, right ; I-V, 

 first to fifth cleavages. The top sides of the figures are anterior. (After 

 Vialleton, from Korscheit and Heider.) 



the third and following phases of cleavage towards the animal 

 pole, alternately in a right-handed and a left-handed direction 

 (Fig. 11). In later stages spirally segmenting eggs assume the 

 characters proper to the first and second types or may do so. 



The law of alternation of direction of cleavage at successive 

 divisions, just alluded to, holds good for a considerable time in 



