KOFOID. — LAWS OF CLEAVAGE. 



195 



Only two spiral cleavages of the seventh generation are figured. 



It will be noted that in Plate II. Fig. 10, the order in position 

 of the quartets of the sixth generation taken from the vegetative 

 toward the animal pole is as follows: a 61 -rf 61 , a e - 3 -d GS , a 6A -d 6A , 

 and a G - 2 -d i)2 , instead of a 61 -d 6 \ a 62 -d 62 , a 63 -d S3 , and a 6A -d 6A , 

 as in Limax, Plate I. Pig. 2. This change in the succession of 

 quartets is due to the fact that in Neritina the quartet a 52 - d 52 by 

 virtue of the spiral occupies a position in the furrows between the 

 macromeres a 6i -d sl . Therefore, when the cleavages resulting in 

 the sixth generation take place, the upper derivatives (a 6 - 2 - d 62 ) of 

 the quartet a 51 -d 51 come to occupy a position nearer the animal 

 pole than the upper derivatives (a 6A -d 6A ) of the quartet a 52 -d 5 - 2 . 

 The application of the system of nomenclature proposed in this 

 paper is not thereby involved in difficulty, for the designation 

 of the daughter cells is in all cases predetermined by that of 

 the mother cell, according to the basis set forth on page 186, the 

 upper derivative in every case receiving the even exponent and 

 the lower one the odd. Thus, notwithstanding the secondary 



