PLATE 1 
EXPLANATION OF FIGURES 
All figures drawn with camera lucida from sections of normal embryos. 
1 Part of a frontal section of an embryo of R. palustris, 3.6 mm. long. (Series 
5b, Row 3, Section 7.) The section is taken through the dorsal half of the medul- 
lary cord at the level of the dorsal cells of Rohon-Beard. One of these cells is 
shown sending out a branched process inj) into the notch between the ninth 
and tenth myotomes {my^ and myxo). X 465. 
2 Part of a frontal section through the head region of an embryo of R. esculenta, 
3.0 mm. long. (Control Experiment Y, 18 hours, Row 2, Section 1.) The 
figure shows a portion of the trigeminus ganglion {gn) with the fibers of the 
r. ophthalmicus (n/) just beginning to sprout. X 465. 
3 Part of a frontal section through an embryo of R. palustris, 4.2 mm. long. 
(Series 6b, Row 4, Section 2. ) The section is taken at the same level as that shown 
in fig. 1. Four dorsal neuroblasts {nhl) send out long nerve fibers (n/) between 
two myotomes {my-2, and my^). X 465. 
4 Part of a sagittal section through an embryo of R. pipiens (virescens), 4 mm. 
long. (Series 7c, Row 5, Section 14.) The end of one of the dorsal nerves con- 
sisting of several fibers is shown. The nerve ending occupies the space between 
two myotomes {my^ and myio) and a thick ridge of the epidermis (ep). X 465. 
5 End of a similar nerve fiber, from another section of same series as fig. 4. 
Taken between myiz and my^^, Row 3, Section 10. X 930. 
6 Branching nerve between myotomes 4 and 5 from a section of the same em- 
bryo as figs. 4 and 5. The drawing, as indicated by breaks in the fibers, is com- 
bined from two successive sections. (Row 5, Sections 12 and 13.) X 465. 
