KOFOID. — LAWS OF CLEAVAGE. 189 



though all the other cells of that quartet, and all other quartets 

 of that generation, were actually present. Thus in Plate I. Fig. 3, 

 the cell c 6 - 7 is the only one of its quartet actually formed, and 

 only six of the eight quartets of the sixth generation have as yet 

 arisen. The missing cells and quartets are of course represented 

 in their ancestors of the fifth generation. The divisions of the 

 quartets resulting in the sixth generation will be discussed in the 

 order of their occurrence. 



First Quartet. The cells a 51 -d 51 divide, forming a 61 -d 61 and 

 a 6.2_^6.2 (plate I. Figs. 2, 4). The cells of the latter (second 

 exponent = 2) lie above and to the right of those of the former 

 (second exponent = 1) with which they are associated (Plate I. 

 Fig. 2). Therefore the spiral is a right spiral. In this genera- 

 tion, as in the preceding, it will be observed that the basal quartet 

 is the first to divide. 



Second Quartet. The cells of the quartet a 5 - 2 -d 5 - 2 divide, form- 

 ing a es - d 63 and a 6A - d 6i (Plate I. Figs. 2, 3, 4), and, as in the pre- 

 ceding quartet, every cell with an even exponent lies above and to 

 the right of the one which was cognate with it, and which has an 

 odd exponent. Therefore this spiral is also a right spiral. This 

 division follows immediately upon that of the preceding (first) 

 quartet, and results in the twenty-four-cell stage of Figure 2 

 (Plate I.). In this stage the embryo is composed of sixteen cells 

 of the sixth generation and eight of the fifth. 



The Fourth Quartet (a SA - d 5A ) divides, forming a 67 - d M and 

 a 6s - d 6,8 , Plate I. Fig. 3. Here, as in the other divisions resulting 

 in the sixth generation, the upper derivatives lie to the right of the 

 corresponding lower derivative, and the spiral is a right spiral. 

 It is a matter of importance to note that the quartet a 5A -d 5i , 

 whose cells are larger than those of its sister quartet a 53 -d 5 - 3 , 

 divides before the latter quartet does. Plate I. Fig. 3. 



Third Quartet. The division of a 53 -d 5 - 3 results in forming 

 a 6.5_^6.5 anc i a 6.6_^6.6_ xhis, like the other three, produces a right 

 spiral. The division of this third quartet is begun before that 

 of the fourth quartet is completed ; likewise, before the comple- 

 tion of the division of this last quartet of the sixth generation, 

 that resulting in the seventh generation begins, and this so far as 

 followed has left spirals. Thus we see that in the cleavage of 

 Limax there exists an alternation in the direction of the s]rirals in 

 successive generations. The spirals of the even generations are 

 right spirals, and those of the odd generations are left spirals. 



