244 OUTLINES OF CHORDATE DEVELOPMENT 



the central portion of the germ disc has become divided hori- 

 zontally by a cleavage plane, connecting the lower margins of 



FIG. 89. Cleavage in the hen's egg. Surface views of the blastoderm and 

 the inner part of the marginal periblast only. From Patterson. The anterior 

 margin of the blastodisc is toward the top of the page. A. Two-cell stage. 

 About three hours after fertilization. B. Four cells. About three and one-fourth 

 hours after fertilization. C. Eight cells. About four hours after fertilization. 



D. Thirty-four cells. About four and three-fourths hours after fertilization. 



E. One hundred and fifty-four cells upon the surface; the blastoderm averages 

 about three cells in thickness at this stage. About seven hours after fertiliza- 

 tion, ac, Accessory cleavage furrows; ra, radial furrow; p. inner part of marginal 

 periblast; sac, small cell formed by the accessory cleavage furrows. 



the first and second planes. In this way a small group of 

 central cells become completely circumscribed, and are to be 



