12 EMBEYOLOGY OF THE LOWEB VEETEBEATES oh. 



furrows which intersect a or (Z at some distance from the poles. As 



regards the variation in order of development of the various furrows 



a good idea will be got from Fig. 6. 



Whatever be the case with the divisions immediately succeeding 

 the eight cell stage, from now onwards there 

 is little regularity. All that can be said 

 is that each individual blastomere goes on 

 dividing over and over again, the length of 

 time elapsing between successive divisions 

 bearing a rough relation to the amount 

 of yolk present in the particular blasto- 

 mere. 



Already at the third cleavage the eight 

 blastomeres have a distinct chink — the com- 

 mencing blastocoele — between their inner 

 ends and as segmentation goes on this space 

 becomes larger. The thirty-two-cell stage is 

 a .blastula which in a meridional section (Fig. 

 7, A) is seen to correspond in its general 

 character with the blastula of Amphioxus 

 but to differ from it in three features : 

 (1) it is of larger size, (2) it is composed of 

 fewer cells and (3) the difference in size 

 between the less richly yolked cells towards 

 the apical pole and the more heavily yolked 

 cells towards the opposite pole is more 

 marked. 



As development proceeds a farther differ- 

 ence becomes apparent. In the various 

 mitotic divisions during the preceding 

 phases of segmentation the axis of the spindle 

 has been arranged more or less tangentially 

 but now divisions begin to take place in 

 which the spindle axes are arranged radi- 

 ally and the division -planes tangentially. 

 When this happens one of the two result- 

 ing daughter cells is nearer the centre, 

 the other nearer to the surface of the 

 blastula and the effect of repeated divisions 

 of this type is that the blastula-wall loses 

 its original character of being composed 

 only of a single layer of cells and becomes 



several cells thick (Fig. 7, B). 



Fig. 6. — Illustrating the varia- 

 tion in the order of appear- 

 ance of the first cleavage 

 furrows in Ran a palustris. 

 (After Jordan and Eycles- 

 hymer, 1894.) 

 The sequence, in time, of the 

 appearance of the furrows is in- 

 dicated as follows:—!, ; 



5, • •• . 



ELASMOBEANCHS 



The egg of any ordinary Elasmobranch such as a Dogfish, Skate, 

 or Torpedo, illustrates the type of segmentation that takes place 



