MICRODISSECTION STUDIES. 



337 



layer in a segmented egg bridges the segmentation furrow. When 

 the furrow is first formed, however, the hyaline plasma layer does 

 not bridge the furrow, but is carried in on the walls of the cleavage 

 furrow (Fig. 20-0, b, r). The layer is thicker in the floor of the 



Fgf-20 



FIG. 20. Contour of a sand-dollar egg at various stages of its cleavage 

 into two blastomeres. In a and b the hyaline plasma layer is seen carried in 

 on the walls of the deepening furrow. In c the egg has segmented in two 

 with the hyaline plasma layer on opposite sides of the furrow tending to 

 merge into each other. In d this process is carried further. In e the two 

 blastomeres are tending to assume the shape of hemispheres with the hyaline 

 plasma layer bridging the furrow. 



furrow, but it is only later when the furrow has cut through the 

 egg that the hyaline plasma layers on the opposite surfaces of 

 the furrow run together. Each half of the segmenting egg tends 

 to assume the shape of a sphere owing to the separation of the 

 two .asters of the amphiaster (Chambers, 'ij-b, '19). If there 

 were no other forces at play, the two blastomeres, when formed, 

 should be spheres. In the sea-urchin egg the adhesiveness of the 

 hyaline plasma layer tends to draw the two blastomeres together; 

 also the fertilization membrane, not rising to any great extent off 

 the surface of the egg, must exert some pressure on the two blas- 

 tomeres. In the sand-dollar the fertilization membrane is well 



