424 MORGAN. [Vol. X. 



appearance of the eye-evaginations on each side (see Figs. 7, 

 B, and 8, B). The posterior-end of the younger embryo is 

 much wider than that of the older, and forms in the younger 

 embryo the characteristic tail-knob (Figs. 5, C, and 6, Q. A 

 comparison of the two stages under consideration brings out a 

 most important relation. Before discussing this I should like 

 to refer to other figures of the same stages that have been pre- 

 pared by another reagent. These figures are shown in Figs. 

 7, A, B, C, and 8, A, B, C. We see that they have contracted 

 less in hardening than the preceding embryos (osmic). Never- 

 theless, these (Perenyi's fluid) have retained the same relative 

 proportions that the former showed. 



One must conclude from an examination of these stages that, 

 while the breadth (surface) of the two stages is the same, the 

 older embryo has a much less depth and a greater elonga- 

 tion. The conclusion necessarily follows that between these 

 stages ati elongation takes place at the expense of the depth. 



A study of a series of cross-sections at the several stages 

 described above gives additional data as to the method of the 

 formation of the embryo. 



A cross-section through the middle of the blastoderm before 

 the thinning out has commenced is shown in PI. XXIV, Fig. 

 IS, A; and a more magnified portion of the same is drawn 

 in Fig. 13, r. 



A cross-section through the apex of the blastoderm (at 1 1 a.m.) 

 is shown in PI. XXIV, Fig. 14, B. The apex has at this stage 

 become thinner. A longitudinal median section through the 

 same stage is shown in Fig. 14, A. A cross-section through the 

 embryonic portion at this stage shows that it contains a large 

 amount of embryonic material, and that in height it is scarcely 

 less than the average thickness of the preceding stage. Fig. 



A series of three figures through an embryo (at 2.10 p.m., an 

 hour older than that of Fig. 3, A) are shown in Figs. 9, A, B, C. 

 Forty sections pass through the embryo, and the first drawn is 

 the seventh of the series. In this figure the embryonic portion 

 is found to have thickened in the axial line. The next figure, B, 

 is the 13th section ; the third figure, C, is the 22d section, and 



