1"' - DEVELOPMENT. 



199. Segmentation. Draw two-cell, four-cell, eight-cell, and morula stages in the 

 development of the egg of a sea-urchin. Note the fertilization membrane. 





200. Blastula. Draw the blastula stage of a sea-urchin, showing : single layer ol 

 cells, thicker at one pole ; cilia ; blastocoele ; mesenchyme cells. 



201. Gastrula. Draw the gastrula stage of a sea-urchin, showing : ectoderm ; 

 endoderm ; mesenchyme ; archenteron ; blastopore ; blastocoele. 



202. Older Gastrula. Make a drawing showing, in addition to the above : coelomic 

 pouches. 



