FIG. 92. Normal development of the sea-urchin, Strong ylocentrotus 

 lividus. (After Boveri, 1901.) 



The animal pole is uppermost in all cases, and in the first two figures 

 the jelly with the canal (micropyle) is shown. 



a, primary oocyte ; the pigment is uniformly peripheral. 



I, ovum after extrusion of polar bodies: the pigment now forms 

 a subequatorial band. The nucleus is ex-axial. 



c, d, first division (meridional). 



e, 8 cells, the pigment almost wholly in the vegetative blastomeres. 



/, formation of mesomeres (animal cells) by meridional division : the 

 vegetative cells have divided into macromeres and micromeres. 



/, blastula. h, mesenchyme blastula. 



i, j, k, invagination of the pigmented cells to form the archenteron of 

 the gastrula. In .7' the primary mesenchyme is separated into two 

 groups, in each of which, in k, a spicule has been secreted. In k the 

 secondary, pigmented mesenchyme is being budded off from the inner 

 end of the archenteron. 



