PLATE 1 



EXPLANATION OF FIGURES 



All figures were drawn with the aid of the camera lucida. Higgin's carmine 

 and true blue inks were used to reproduce the colors of the stained sections repre- 

 sented in the colored plate. 



1 Lymphoid accumulation in the lateral branchial wall of a 63 mm. larva. 

 Ep., Endodermal epithelium; End., endothelium of blood sinus; pb.b.s., peri- 

 branchial blood sinus; trab., connective tissue trabecula in blood sinus. dV 

 oil immersion obj., ocular No. 3.) 



2 Portion of a frontal section of a 5 mm. larva showing the epithelial placode 

 in the lateral branchial wall. Ect., ectoderm; End., endoderm; Ales., mesen- 

 chyma. (iV oil immer. obj., ocular No. 3.) 



3 Epithelial placode in lateral branchial wall of a 9.5 mm. larva. Frontal 

 section. Am.n., amoeboid nuclei; b.c, red blood cells; bn., longitudinal muscle 

 fibers; tm., transverse muscle fibers. (yV oil immer. obj., ocular No. 3.) 



4 Epithelial placode in lateral branchial wall of a 15 mm. larva. Frontal 

 section, pb.b.s., peri-branchial blood sinus; s.l., layer of flat cells forming at 

 surface of placode. (^ oil immer. obj., ocular No. 3.) 



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