CLEAVAGE 



233 



FIG. 109. Cleavage in the sea-urchin, Strongylocentrotus lividus. From 

 Jenkinson, after Boveri. Animal pole uppermost in all cases, a. Primary 

 oocyte surrounded by jelly, and containing large germinal vesicle with nucleolus. 

 Pigment uniformly distributed over surface, b. Ovum after formation of polar 

 bodies. Pigment forms a band below the equator, c, d. First cleavage, e. 

 Eight-cells. Pigment almost wholly in lower quartet (vegetative blastomeres) . 

 /. Sixteen-cells. The lower quartet has divided latitudinally and unequally, 

 forming four micromeres at the vegetal pole; the upper quartet has divided me- 

 ridionally forming a plate of eight cells, g. Section through blastula. h. Later 

 blastula, showing formation of mesenchyme at lower pole, t, j, k. Three stages 

 in gastrulation, showing the infolding of the pigmented'cells to form the endoderm 

 (archenteron) . In j the primary mesenchyme is separated into two masses, in 

 each of which a spicule is formed (k). In k the secondary, or pigmented, mesen- 

 chyme is being budded off from the inner end of the archenteron. 



