4 INTRODUCTORY I 



subequatorial position). The second divisions, or more properly 

 the two divisions of the second phase, are again meridional, 

 and there are now four blastomeres, each of which has a 

 similar share of the three regions of the egg. But at the 

 next cleavage, which is equatorial, four unpigmented animal 

 cells are separated from four vegetative cells, each of which 

 has one quarter of the pigmented ring and of the smaller 

 unpigmented area. 



The direction of the divisions of the fourth phase is different 

 in the animal and in the vegetative hemispheres. In the 

 former it is meridional, producing a ring of eight cells, while 

 in the latter it is latitudinal (parallel to the equator) and 

 unequal, resulting in four large blastomeres (macromeres), 

 which take the pigment, and four small ones, the micromeres, 

 round the vegetative pole. 



Division proceeds with regularity and synchrony, at least 

 in some regions of the ovum, for some little time yet, but we 

 need not follow the details. Eventually the divisions become 

 irregular. The final result of segmentation is the bias tula, or 

 hollow sphere of cells, disposed in a single layer round the 

 central blastocoel or segmentation cavity. The cells are 

 ciliated, and the blastula escapes from the egg-membrane. 

 In the blastula the same three regions are present as in the 

 unsegmented egg, the large unpigmented, the pigment ring, 

 and the small unpigmented. Segmentation has therefore 

 merely cut up the unlike material of the egg into small 

 pieces, and beyond the segmentation cavity and the cilia 

 there has been formed no new structure, there has been no 

 differentiation. 



The next step is the development of the primary mesen- 

 chyme. One by one the cells of the small vegetative un- 

 pigmented area become amoeboid and migrate into the 

 blastocoel. There they quickly arrange themselves in two 

 groups, of which one lies upon the right, the other upon the 

 left, of the future median plane. Each group secretes a tri- 

 radiate calcareous spicule, which is the rudiment of the 

 skeleton of the Pluteus larva. The two groups are connected 

 by two curved lines of cells, one of which, on the future 



