454 HOLMES. 



EXPLANATION OF PLATE XIX. 



Fig. 25. Apical view of an egg of 104 cells. The trochoblasts have increased 

 in size, become clearer, and have apparently compressed the arms of the cross 

 which stand out in greater contrast to the cells between them than in the earlier 

 stages of development. 



Figs. 26, 27, 28, and 29. Lateral views of the same egg. It will be seen that 

 the cells of the second quartette in the b quadrant have not kept pace. with the 

 divisions of those of the other three quadrants. In Fig. 29, 3a''^ has divided, 

 while its fellow, 3a''', remains entire ; the corresponding divisions have both 

 occurred in the d quadrant. 



Fig. 30. Lower pole of same egg. The cells of the fourth quartette have 

 undergone a second cleavage. 



Fig. 31. Apical pole of an egg of 130 cells. The tip cells of the cross have 

 enlarged and become clear, the arms show a marked laeotropic twist, and the split- 

 ting of the arms is begun by a transverse division of one of the cells of the anterior 

 arm. The apical pole has begun to rotate forward, and the posterior trochoblasts 

 have become slightly larger than the others. 



Fig. 32. Left side of same egg. 



Fig. 33. Anterior side of same egg. The third quartette in the b quadrant 

 consists of a double row of four cells. In the c quadrant the divisions of the third 

 quartette are somewhat more advanced, y^''^'^ has just divided, while the corre- 

 sponding cell, 3f'^-''S is entire. 3c'-2-2 has divided, and its fellow, 3<^'•'•^ is undergo- 

 ing division. The upper cells of the second quartette are beginning to become 

 clear and to take on the character of the neighboring trochoblasts. 



Fig. 34. Right side of same egg ; the forward rotation of the apical pole may 

 be seen here as in Fig. 32. 



Fig. 35. Posterior side of same egg. 



Fig. 36. Vegetal pole of same egg. A fifth quartette has been formed, and 

 the cells of the fourth quartette in the b quadrant have undergone a division at 

 right angles to the previous one ; the corresponding cells in the a and c quadrants 

 remain undivided. 



