No. 2.] FORMATION OF THE FISH EMBRYO. 425 



is at the middle of the embryo. The axial concentration is 

 scarcely seen in this last section. Owing to the greater area of 

 the last section (9, C), it contains much more material than 

 either of the preceding. 



Three cross-sections through an embryo (at 3 p.m., see Figs. 

 4, Ay B) are shown in Figs. 10, A, B, C. We find that at this 

 time the axial thickening has progressed more rapidly than in 

 the preceding stage, and extended farther backwards. The first 

 section, A, is the 7th of the series; the second is the 20th. 

 This series had 52 sections. The third figure, C, is from 

 another embryo, and lies near the center of the embryo. Com- 

 paring it with Fig. 9, C, we find it thicker, but narrower. 



Serial cross-sections through an older embryo (at 5.30 p.m., 

 see Fig. 5, A) are drawn in Figs. 11, A, B. The first of these 

 is the 1 8th section. The second is from another series of 116, 

 and is the 85th section, and lies, therefore, 31 sections in front 

 of the posterior end. Great care was taken to get these sec- 

 tions exactly transverse to the long axis. These sections show 

 a much greater axial thickness, and a correspondingly less 

 surface exposure than the last. 



The series of cross-sections drawn in Figs. 12, A, B, C, D, 

 come from embryos (at 7.05 p.m., see Fig, 6, A). The first of 

 these, A, is the 26th, the second, B, is the 3 2d. Figures C 

 and D are from another embryo. Of these, C is the 35th 

 section anterior to the posterior end, and D the 13th in front 

 of the posterior end. 



Comparing the last two sets of figures. Figs. 11 and 12, we 

 find that, while the surface (dorsal) exposure is about the same 

 in both sets, the depth of the younger series is greater than that 

 of the older. This is true, both for the anterior and posterior 

 parts of the body, and is seen most strikingly when more en- 

 larged figures, of the two stages, are compared. This result 

 agrees with that noticed in surface preparations and optical 

 sections. 



A volumetric comparison of all of these sections from corre- 

 sponding regions of the different embryos would be most valuable, 

 because we could then determine whether all of the area of the 

 younger and wider sections was greater, or equal, or less, than 



