IV. i INITIAL STRUCTURE OF THE GERM 191 



and by division of these a hemispherical curved plate, a half- 

 blastula(Fig.98). When, as may happen, the other blastomere dies 

 without being separated, it is embraced by the open side of the 

 survivor. Only after the 32-celled stage is passed do the edges 

 meet and close. A blastula of half-size is the result, which 

 becomes later a gastrula and a Pluteus of perfectly normal form 

 but of half the normal size. 



a 6 c d 



FIG. 99. Segmentation of an isolated \ blastomere of Echinus; a, j, 

 & f > c i T%> one micromere, one macromere, two mesomeres, d, wV. (After 

 Driesch, 1893.) 



| blastomeres behaved in the same way, segmenting as parts, 

 developing ultimately into whole normal Plutei. Similarly blasto- 

 meres segmented partially (Fig. 99), with, in some cases, irregu- 

 larities : two micromeres, for instance, were formed in the second 

 division. The blastulae will gastrulate, but progress no further. 



A B 



FIG. 100. A, Gastrula with mesenchyme cells and triradiate spicules 

 reared from a J animal cell of Sphaerechimts. B, The same from a % cell 

 of Echinus : the gut is tripartite. (After Driesch, 1900.) 



In the next stage | blastomeres a difference becomes notice- 

 able between the behaviour of the animal and vegetative cells. 

 Both kinds of cells will gastrulate, secrete triradiate spicules and 



