150 IVAN E. WALLIN 



From the above described transitional conditions and from a 

 study of the stained sections, I can draw no other conclusion 

 than that, the nuclei of the epithehal placode transform into 

 lymphocyte-like cells. It is a significant circumstance that the 

 nuclei alone migrate from the placode, i.e., no cytoplasm is vis- 

 ible. Complete cells bearing epithelial characters may be found 

 in the connective tissue spaces. However, I have never found 

 them migrating from the placode while I have found migrations 

 of the complete cell from the epithelium of other regions. 



Cells in mitosis may be seen occasionally in the placodes of 

 the 31 mm. larva. Cells in a state of amitotic division, how- 

 ever, are quite abundant in a 44 mm. larva, suggesting that 

 cell-proliferation takes place chiefly by simple fission. Figure 

 15 shows the nucleus of a placode cell apparently in a process of 

 simple fission. Mitotic cells are especially scarce in the con- 

 nective tissue above the placode. A single instance has been 

 found and is represented in figure 18. It is quite evident from 

 the lack of mitotic or amitotic cells or nuclei in the connective 

 tissue that cells or nuclei are not being formed in any significant 

 quantities in this situation. 



Transformation stages have not been found within the plac- 

 ode in the 31 mm. larva. Lymphocyte-like cells, however, are 

 present in the placodes. Their presence may be accounted for 

 by means of an immigration from the connective tissue. 



The further development of the placodes is a repetition of the 

 above-described processes except that the transformation is more 

 rapid and begins within the placode. Figure 8 represents a part 

 of a transverse section of the placode and the connective tissue 

 above it in a 44 mm. larva. The illustration was drawn to the 

 same magnification as figure 7. The nuclei in the placode are 

 elongated and show amoeboid characters. They also appear 

 to be in an active state of emigration. The transformation 

 process appears to have begun in the placode in this stage. 

 The nuclei near the connective tissue border have taken on 

 characters which approach the characters of some of the nuclei 

 in the connective tissue. This change is shown in the staining 

 reaction, the condition of the chromatin, and the shape of the 



