148 IVAN E. WALLIN 



form a loose mesh work. Figure 7 represents a transverse sec- 

 tion through the cephalic part of a placode with its neighboring 

 connective tissue and the ciliated epithelium of one side. In 

 such a region the nuclear elements are very scarce in the con- 

 nective tissue when compared to the region above the central 

 part of a placode. The basement membrane which is present only 

 on the right-hand side in the section illustrated in figure 7 (b.m.) 

 bridges across the entire placode a few sections cephalad of the 

 one illustrated. In the central part there is no line of demarca- 

 tion between the placode and the connective tissue. The cyto- 

 plasm of the placode in this place is directly continuous with the 

 connective tissue. 



The cells within the placode have greatly increased in num- 

 bers in this stage of development. Near the free surface of the 

 placode they are loosened from each other, displaying their 

 rounded outlines distinctly. Toward the deeper part of the 

 placode the cells become oblong in shape. Near the connective 

 tissue border the cell outlines are lost which gives the appear- 

 ance of a syncytium. The nuclei of the cells in the placode are 

 not unlike the nuclei of the ciliated and the general branchial 

 epithelium in their general morphological characters, except in 

 size. They are smaller than the nuclei of the ciliated and gen- 

 eral epithelium. The chromatin of the nuclei, for the most part, 

 is collected into a single lump which stains a reddish-purple 

 with the haematoxylin-eosin stain. The nuclei also change 

 from a circular to an oblong outline from the free surface of the 

 placode to the connective tissue border. At the place where 

 the cytoplasm of the placode is continuous with the connective 

 tissue, the nuclei become quite elongated, having the appear- 

 ance suggestive of a migration into the connective tissue. This 

 migratory appearance is more prominent at the central part of 

 the placode. Figure 10 represents a part of a transverse sec- 

 tion from the central region of the placode. The two lower 

 nuclei marked a in the figure lie in the cytoplasm of the placode. 

 All the nuclei and cells above this level are in the connective 

 tissue and spaces above the placode. The nuclei in the connec- 

 tive tissue show degrees of gradual variations in morphological 



