658 



BIRDS. 



a. p. 



Al 



FIG. 328. Stages in development of chick. After Marshall. 



1. Segmentation, superficial view of blastoderm. 



2. Vertical section of blastoderm. Ep., Epiblast ; I.e., lower layer of 



cells ; s.g.c., sub-germinal cavity ; y., yolk. 



