20 



at right angles to the first. Sometimes tiie second furrow 

 starts iyrregularly , at a point part way around the egg from 

 the origin of the first furrow. One of the hemispheres is 

 thus divided before the other, as in figure 6. "Fifty min- 

 utes elapse between completion of the first and second fi^r- 

 rows. Suc':'essive segmentations come in at intervals of 45 

 to 60 minutes. 



With the eight-celled stage rotation of the blasto- 

 meres occurs. The four upper ceils turn through an angle 

 of Ard^wpon the lower ones, so that they come to lie in the 

 valleys between the four lower ones instead of being exact- 

 ly superimposed upon them. Segmentation continues until a 

 hollow blastula (fig. 7) is produced, a layer of thick 

 cells surrounding a small cleavage cavity. The cells are 

 of unifonn thickness, and their outer ends give rise to 

 cilia which drive the egg round and round by their motion, 

 within the membrane, sometimes in one direction, sometimes 

 in the other. During this stage the fo rma tion of the endo- 

 derm takes place. The inner ends of the blastoneres are 

 delaminated, the process going on at an equal rate on all 

 sides, until a unifo):™ layer of endoderm cells lies within 

 the ectodermal layer ("Fig. 8). By increase in size of these 

 endodermal cells the cavity of the egg is entirely obliter- 



