258 T. H. BurLenp, 
Explanation of Plates. 
The left side of each of the Plate figures is the right side of the 
embryo and vice-versa. 
List of reference numbers: 
1. spinal cord 9. ectoderm 
2. notochord 10. endoderm 
3. dorsal aorta 11. nephrostome 
4. protovertebra 12. primitive kidney groove or rudi- 
5. nephrotome ment 
6. coelom 15. glomus 
7. somatic mesoderm 14, pronephric artery 
S. splanchnic mesoderm 15. nephrocoel 
Plate 18. 
Fig. 1. Transverse section through embryo A in the region of 
the front end of protovertebra IX. It is the section 2,6 in the 
text. Notice the protovertebra, the nephrotome and the lateral plate 
regions: the upper portion of the latter is becoming differentiated to give 
rise to the primitive kidney groove, which is fairly distinct on the left 
side of the figure. 
Figs. 2—18 are photographs of successive sections of embryo B in the 
region of protovertebrae VII, VIII and IX. 
Fig. 2 (Section 1,152). Passes through protovertebra VII of each 
side. Notice that on the left side of the figure the kidney rudiment forms 
the swollen upper end of the lateral plate region. On the right side the 
myocoel, nephrocoel and coelom are in communication. The nephrocoel 
