196 TREAD WELL. [VOL. I. 



sions are dexiotropic. The third group of micromeres appears, 

 and at the same time cells a 1 ' 2 , b 1 ' 2 , c 1 ' 2 , d 1 ' 2 are formed ; a 1 ' 1 , 

 b lml , c 1 ' 1 , d 1 ' 1 then divide to form the primary trochoblasts, and a 

 little later the 32-cell stage is completed by the division of the 

 second group of micromeres. 



From thirty-two to sixty-four cells all divisions are laeo- 

 tropic, and this period may be divided into three stages. 



First, from thirty-two to forty cells. This is accomplished 

 by the division of A 3 , B^ C 3 , D 3 , to form the fourth group of 

 micromeres, and the formation of the apical rosette by the 

 division of a 1 ' 2 , b 1 ' 2 , c 1 ' 2 , and d 1 ' 2 . 



The micromeres of this fourth quartette are equal in size, 

 and lie between the cell from which they arose and the left- 

 hand descendant of the second group of micromeres of the 

 corresponding quadrant. At this stage, since the cells given 

 off from the four cells at the top have been smaller than the 

 micromeres of the second and third generations, and since the 

 apical rosette cells are very much smaller than the cells of the 

 fourth generation of micromeres, it follows that the four cells 

 surrounding the rosette are much the largest in the embryo. 

 These cells later divide and form the prominent cross. (See 

 Fig. 4.) The rosette cells elongate and push into the segmen- 

 tation cavity, retaining their connection with the outside by 

 only a slender stalk. They later come to the surface and 

 divide. (See Fig. 3, where is shown also the relative size of 

 cells at the two poles of the embryo ; those at the upper pole 

 are dividing to form the cross.) 



Second, forty to fifty-six cells. This is accomplished by a 

 division of cells a 1 '", ''', c l ' l '\ d 1 '"; a 1 ' 1 ' 2 , 6 1 - 1 *, c 1 ' 1 ' 2 , d l ' l2 \ a 1 ' 2 , 

 b 1 ' 2 , c 1 ' 2 , d 1 - 2 ; and a 3 , b\ c\ d 3 . The first two of these sets of 

 cells are the primary trocJioblasts. By their division are 

 formed sixteen cells, all of which acquire cilia and become the 

 primary prototroch, which here, as in other forms, is composed 

 of four distinct areas of ciliated cells. These later, by the 

 addition of cells from the lower hemisphere, become united 

 into a single band with a dorsal interruption. Before the com- 

 pletion of the prototroch band a strong tuft of cilia is formed 

 at the apical pole. 



