PLATE 1 



EXPLANATION OF FIGURES 



1 Epithelial extensions from a piece of the tail of a Diemyctylus larva a week 

 after implantation. The darker sheet of ectoderm represents the part extending 

 in contact with the cover slip of the hanging drop culture; the lighter lower ex- 

 tension was in contact with the lower side of the drop. 1^ represents the original 

 piece which was taken from a young larva before it had hatched from the jelly; 

 it was quite opaque and showed no chromatophores which had become clearly 

 differentiated a week later. 



2 Epithelial extensions from a piece of tissue which had been cultivated 

 for over two months, subdivided once, and subcultured several times. The dark 

 central part represents the piece from which two large sheets of epidei-mis, an 

 upper and a lower, have extended. 



3 A multinucleated cell of epidermis. 



4 Granular cell of the epidermis. 



5 Two alveolar cells of the epidermis which differentiated in vitro. 



6 Ordinary epidermal cell. 



7 Epidermal cells from the edge of an advancing sheet of epidermis in con- 

 tact with the cover slip. The thin hyaline border shows numerous fine pseudo- 

 pods in contact with the glass. 



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