BEHAVIOR OF CELLS IX FUXDULUS HETEROCLITUS. 225 



cells at the edge of the ectoderm, hut the presence of such granules 

 in the flat ectodermal cells could not be determined with cer- 

 tainty. These cells appeared less easily penetrable to a number 

 of vital dyes, while the mesenchyme cells were readily colored. 



Formation of Ectodermal Membrane. In a study of fetal skin 

 growing in blood serum, Loeb ('12) observed that the ectoderm 

 cells migrated into the surrounding medium in the form of strands. 

 Holmes ('13) and Uhlenhuth ('14) described a similar condition 

 in the frog. In my observations on fundulus the ectodermal 

 outgrowths were invariably in the form of very thin, one-layered 

 membranes. The ectoderm never migrated in strands. The 

 earliest appearance of the membrane was indicated by an exceed- 

 ingly thin, flat layer near the explant which was continuous with 

 the rounded cells covering the body region, and bordered along 

 its outer edge by a broad mass of irregular thickened cells. The 

 mesenchyme cells lay in a thin sheet above the ectoderm and 

 projected beyond it. As the rounded cells migrated from the 

 explant their contour changed gradually, and they became 

 flattened w r ith irregular, thickened, central portions which pro- 

 jected downward in the fluid medium. Text-figure I is a drawing 

 of an ectodermal cell in the process of flattening out during its 

 migration. The portion of the cytoplasm spread out along the 

 cover-glass formed a clear thin area bounded by a cell wall which 

 was in close contact with the walls of neighboring cells. In this 

 clear region were a few pale concentric markings. The central 

 mass of thickened cytoplasm eventually disappeared as the cell 

 flattened out completely. A few hours later, as a result of the 

 migration and flattening of the cells, the membrane had increased 

 more than twice in extent, and no thickened cells remained 

 except a few scattered ones and a single row of elongated cells 

 around the edge. 



Changes at the Edge of the Ectodermal Membrane. The edge 

 of the ectodermal membrane was seen to undergo slow and 

 continual changes during the active growth of the culture. 

 Usually the cells were thickened and elongated or extremely 

 irregular in form, with numerous blunt knobs projecting down- 

 ward. Their cytoplasm appeared granular in contrast to the 



