564 KNOWER. 



EXPLANATION OF PLATE XXX. 



Fig. II*. Dorsal surface. The cells at posterior end of this surface crowded 

 closely together to form posterior border, p.b.d. (posterior limit of disc) of the 

 area ca. of ventral surface. The collection of nuclei forming this border are in 

 sharp contrast to those scattered over this surface. 



Fig. lib. Side view, giving better idea of how markedly the cells on the sur- 

 faces of the egg have crowded back to the region marked ca. in figures : p.l.d., 

 posterior limit of cap or disc well shown ; a.l.d. anterior limit of disc ; A. and P., 

 anterior and posterior. 



Fig. 12. Ventral surface of older egg with disc of nuclei forming area ca. 

 about equally distributed to its borders. 



Fig. 13. Further contraction of disc ; all of its boundaries now well within 

 limits of ventral surface : l.b.d., lateral border of disc. 



Fig. 14. Ventral surface of last, seen slightly on one side to show outlines of 

 contracting area. 



Fig. i 5. Posterior end of egg tipped up to show an especially marked concen- 

 tration of embryonic disc. 



Fig. 16. Ventral surface of older germ-disc torn off with a piece of the 

 chorion : ch., chorion ; ul.p., under-layer plug. 



Fig. 17. Ventral view of egg showing germ-disc with crowded posterior mar- 

 gin : am.t., amnion thickening, later to fold forward ; ul.p., under-layer plug more 

 distinct. 



Fig. 18. Ventral surface showing am.t., amnion thickening, at its maximum of 

 crowding before folding over. 



Fig. 19. Older egg, somewhat on side to show amnion fold, am. The ttl.p., 

 extensive under-layer plug. Cells of serosa, and middle thin region of disc, faint. 

 Fig. 19a. Same stage dissected off to show details. Amnion fold reaches for- 

 ward on sides anterior to ul.p. Anterior limit of disc, a.l.d. Note dividing 

 nuclei, straight black rods. 



Fig. 20. Side view of amnio-serosal fold, am., soon after its origin. Cells of 

 embryo not shown : sc, serosa cells posterior to fold ; em.d., embryonic disc ; sea., 

 serosa cells on yolk anterior to embryo ; ch., chorion. 



Fig. 21. Side view of amniotic fold (partly in optical section) half covering 

 the disc: am., amnion, of same appearance as ectoderm of disc, being several 

 layers thick; am.s., side view of amnion; my., two micropyles in distended, 

 wrinkled chorion, ch. 



Fig. 22. Optical section from side of closing amnion ; o.amc., opening from 

 exterior into amniotic cavity ; sc, serosal cells, large and faint on surface of yolk ; 

 yc, yolk<ells deeply stained and lying within the yolk. Cells of embryo and 

 amnion not shown. 



Fig. 23. Same stage as foregoing seen from surface. Lettering as before. 



