224 PAULINE H. DEDERER. 



even when not cut, although the other cells of the culture exhibit 

 normal activity. 



Ectodermal Membrane. Fig. I shows the extent of a charac- 

 teristic membiane in a seven-day culture. In the living cultures 

 the ectodermal cells spread out in a very thin, flat and colorless 

 layer on the under side of the cover-glass. The position of the 

 nucleus and of the granules within the cell could rarely be de- 

 tected. At the edge of the membrane the cells were thicker and 

 darker in appearance, with a greenish tinge and of very irregular 

 form. Projecting beyond the ectoderm, mesenchyme cells could 

 be seen adhering closely to the cover-glass. Other cells, slightly 

 darker and containing vacuoles and granules, were observed 

 migrating above the ectodermal layer. Slides stained with iron 

 hsematoxylin show that such outgrowths from the explant con- 

 sist of a practically continuous ectodermal membrane of extremely 

 flat, slightly granular cells, and a more or less imperfect mem- 

 brane of mesenchyme lying above it, closely adhering to the 

 cover-glass (Fig. 2). The mesenchymal membrane is never so 

 perfect as the ectodermal layer, and there is a gradual transition 

 from a membranous form to more or less isolated cells which 

 may project for a considerable distance beyond the ectoderm 

 (Fig. 3). One group of ectodermal cells was found which was 

 not covered on its upper surface by mesenchyme (Fig. 8). These 

 cells were characterized by very granular nuclei containing one 

 or more nucleoli, and by mitochondria in the form of threads and 

 granules. Frequently the cells showed a rosette-like arrangement 

 around a small intercellular space. Mesenchyme cells were 

 readily distinguishable in the stained slides by their more granular 

 cytoplasm and indefinite cell boundaries. The size of the nuclei 

 was not a criterion, as the nuclei might be either larger or smaller 

 than those of the ectoderm. Mitochondria were much more 

 abundant than in the ectoderm and stained more deeply. Fig. 9 

 shows a group of mesenchyme cells, unaccompanied by ectoderm, 

 in which the mitochondria appear very clearly. They were also 

 observed in the living cells when stained with janus green. 

 Certain other granules became visible when stained with neutral 

 red. The latter stain also affected granules in the thickened 



