FIG. 12. Normal development of the sea-urchin Strongylocentrotiis 

 llc'ulus. (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. 



, primary oocyte, the pigment is uniformly peripheral. 



6, ovum after extrusion of polar bodies. The pigment now forma 

 a subequatorial band. The nucleus is ex-axial. 



c, rf, 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 rnicromeres. 



g, blastula. h, mesenchyme blastula. 



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

 the gastrula. In j 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. 



