DESCRIPTION OF FIGURES. 

 Plate I. 



Fig. 1. A median loiigitiuliiuvl section of a blastoderm talven twenty-one 

 hours after fertilization, or twenty hours before laying, x H^. 



Fig. 2. A portion of the anterior half of a median longitudinal section 

 from a blastoderm taken about thirty-three hours after fertilization, or 

 eight hours before laying. It is not so far advanced in its development as 

 blastoderms usually are when taken at this time. See text for description. 

 X 161. (See also Figs. 29 and VI). 



Fig. 3. A portion of the posterior half of the same section as preceding. 

 Compare these two figures as to the condition of the zone of junction, x 161- 



Fig. 4. Posterior end of a section, twenty sections to the right of the pi > 

 ceding. Note especially the cells organizing about the periblastic nuclei 

 at "ce," and also the two degenerating periblastic nuclei dm), x 365. 



Fig. 5. Posterior end of a section, five sections to the right of the pre- 

 ceding. This shows the tip of the right horn of the zone of junction, x 365 

 (see also Fig. VI). 



Fig. 6. A group of six (three shown in the section) cells organized . ^t 

 periblastic nuclei, which are in the central periblast, x 657. 



Fig. 7. A normal periblastic nucleus which was introduced for compari- 

 son with the following figures (8-13). X 886. 



Figs. 8-13. Various stages of degenerating periblastic nuclei, x 886. 



Fig. 14. Posterior end of a longitudinal section taken slightly to the 

 left of the median line. It shows the thin epithelial-like margin just before 

 invagination occurs. At "iii" is one of the few yolk masses that are found 

 at this stage, x 259. (See also Figs. 30 and VII). 



