BEHAVIOR OF CELLS IN FUNDULUS HETEROCLITUS. 233 



or reticular membranes with isolated cells lying beyond. The 

 ectoderm formed a membrane in close contact with the under 

 surface of the mesenchyme. Nerve fibers, pigment cells, and 

 yolk cells from the digestive tract readily migrated out. Peri- 

 stalsis of the intestine, beating of the heart, contraction of the 

 trunk musculature, and movements of the fins, were observed 

 in numerous cultures several days old. 



3. Characteristic of the ectoderm cells were certain delicate 

 striations somewhat concentrically arranged, which formed an 

 intricate pattern over the cell. 



4. The mesenchyme cells are highly amoeboid and possess 

 characteristic fan-like expansions by means of which they adhere 

 to the cover-glass and to each other. 



5. While .the initial stages in the formation of the ectodermal 

 membrane were accomplished by migration and flattening out 

 upon the under surface of the mesenchyme of the cells originally 

 covering the body, the further extension of the membrane 

 involved the formation and growth of new cells and tension 

 exerted by the mesenchyme upon the thick edge of the ectoderm. 



6. Mitosis was observed in several mesenchyme cells but not 

 in the ectoderm, although new cell boundaries appeared from 

 time to time. Frequently ectoderm cells contained two nuclei 

 or one irregularly lobed nucleus. 



7. During all stages in the formation of the ectodermal mem- 

 brane the movement of the cells is a mass movement. Their 

 reactions are much slower than those of the mesenchyme, and 

 are never amoeboid in character. 



8. There is an essential similarity in the outgrowth of the 

 ectodermal membrane and the process of wound-healing in 

 respect to (a) the mass migration of the ectoderm cells, and (b) 

 the stereotropic activity of the cells which is evoked by contact 

 with the mesenchyme. 



I wish to acknowledge my indebtedness, for the valuable 

 suggestions and criticism, to Prof. W. H. Lewis and Mrs. Lewis, 

 of the Department of Embryology of the Carnegie Institution 

 of Washington, under whose direction this work was accom- 

 plished. 



