124 



CONKLIN. 



[Vol. XIII. 



Then the upper cells of the anterior quadrants (3a'-S 3a'% 

 3b'S 3b'-^) divide in a dexiotropic direction into the cells 

 3a''-' and 3a'-'-% 3a'=''' and 3a'-^^ etc. 



There are thus formed two upper, two middle, and two lower 

 cells in each of the quadrants A and B, and two upper, one 

 middle, and two lower cells in each of the posterior quadrants, 

 C and D (see Diagram 8). These facts are brought together 

 in the following tables, giving the lineage of 3a and 3d ; 3b is 

 precisely like 3a, 3c like 3d. 



34 



109 



3a'Uhper 



3a^ Lo 



iwer 



3a'"- , ., 

 3a''-^ (Middle) 



3a '■^' 



3d' Ujifier 

 3cl^ Lowrer 



3d'-'-0nner)(E) - 

 3ci'-^(0ut£r)-< 

 3 d^- '(Outer) 



3d'-^'- 



4* 



I have never seen the cells 3c'-' and 3d'-' divide, though they 

 become quite large and the corresponding ones 3a'-^ and 3b'-' 

 do divide at the stage shown in Figs. 38 and 40. Every divi- 

 sion of 3d and 3c is bilateral, except the first, which is transi- 

 tional between the spiral and the bilateral. During this time 

 the only other cells of the ectoblast which show bilateral cleav- 

 age are the tip cells in two of the arms of the cross, vis., 2c'-' 

 and 2d'-'. 



This is the history of the third quartette as far as I have 

 followed it. It is particularly notable, in that bilateral sym- 

 metry within the ectoblast first appears in these cells, and 

 that while all the other cells are dividing spirally the cells of 

 this quartette in the two posterior quadrants always divide 

 bilaterally. 



