380 



AN INTRODUCTION TO ZOOLOGY 



than the other, and cuts off four smaller cells at -the animal pole 

 from four larger ones at the vegetative pole. The eight cells are 



XII 



FIG. 128.' A mphioxus, early cleavage. I. VII., and XII., adapted from 

 Ziegler, after Hatchek. VII.-XL, adapted from Morgan. 



I., ovum unsegmented ; II.-VI., first five cleavages ; VI. and VII., blastula stage ; VIIL, sec- 

 tion of blastula just before gastrulation ; IX. and X., sections of successive stages in gastrulation ; 

 XI., section of fully formed gastrula ; XII., median view of half a late gastrula with neurenteric 

 canal formed. 



A., archenteron ; B., blastoccel ; B.P.,blastopore ; E., ectoderm; En., entoderm ; N.E., neur- 

 enteric canal ; N.P., neural plate ; P.B., polar body. 



turned into sixteen by the simultaneous formation of two meridional 

 cleavages, and then into thirty-two by two latitudinal divisions. 



