4o8 HOLMES. [Vol. XVI. 



before, but this time the division is unequal and gives rise to 

 a small cell lying between the middle and outer cells. The next 

 cleavage occurs in the outer pair of cells and in the same direc- 

 tion as the preceding division. In fact, all the divisions in the 

 mesoblastic bands are henceforth in the same direction, until a 

 considerably later period of development. At the time when 

 there are three cells in each band the mesomeres form a con- 

 cave row of cells in the posterior half of the ^gg. As the 

 bands lengthen by teloblastic budding, they assume the shape 

 of a horseshoe. Later they become resolved into scattered 

 cells. 



The Third Quartette and the Seco7idary Mesoblast. 



At the stage in which the egg contains forty-nine cells the 

 cells of the third quartette are eight in number, arranged in 

 four vertical pairs, lying over the angles between the cells of 

 the fourth quartette. The first cleavage of this quartette forms 

 a transition from the spiral to the bilateral type, and the subse- 

 quent cleavages show a bilateral character in a more marked 

 degree. At nearly the same time the lower pair of cells, "^b^, 

 3^^ in the two anterior quartettes, and the upper pair of cells, 

 3«', 3^', in the posterior quadrants, divide in a nearly horizontal 

 direction into equal moieties. Later, at about the sixty-four-cell 

 stage, the upper pair of cells in the anterior quadrants, 3^5', 3^', 

 divide in the same direction as the lower pair. The lower pair 

 of cells in the two posterior quadrants, 3^^ 3^^ remain undi- 

 vided until a much later stage. In each of the two anterior 

 quadrants there are now two pairs of cells, the one pair lying 

 directly above the other. In each of the posterior quadrants 

 there is a pair of cells lying over a large undivided cell. It 

 is easy to orient the ^^g from the lower pole at this stage by 

 the bilateral arrangement of these cells. Definite spiral cleav- 

 age, which appears in such a marked way in the early divisions 

 of the first and second quartettes, seems entirely absent in the 

 third. The first cleavage is radial, and all the succeeding divi- 

 sions appear to be bilaterally symmetrical with reference to the 

 median plane of the future animal. 



