228 PAULINE H. DEDERER. 



into protoplasmic bridges connecting the cells. When these 

 were present the tissue appeared somewhat similar to the meso- 

 thelial membrane described by Lewis and Lewis ('12) in tissue 

 cultures of the chick, except that in fundulus the cell boundaries 

 are much less distinct. Whether the cells actually fuse or not 

 is a question which is extremely difficult to determine with 

 certainty. W r hen the cells are connected with each other by 

 extremely elongated protoplasmic processes, as shown in Fig. 3, 

 the intercellular spaces are large and a wide-meshed reticulum 

 results. There is a gradual transition from a membranous to a 

 reticular arrangement of cells as the distance from the explant 

 increases. 



The outward growth of these reticular cells appears to be an 

 important factor in the extension of the ectoderm. As previously 

 noted, reticular mesenchyme cells were usually seen projecting 

 beyond the ectodermal membrane (Figs. I and 3). These cells 

 underwent very marked changes in form, their protoplasm flowing 

 in a direction away from the ectoderm, the proximal portions 

 of the cells, however, being connected directly or by intervening 

 cells with the thickened edge of the ectoderm. The distal 

 portions of the cells frequently developed broad fan-like expan- 

 sions, which were firmly anchored to the cover glass. 



The mode of formation of the fan-like expansions appears to 

 be about as follows: The pseudopodia flow out from the cell in 

 the form of delicate, radiating, thread-like or finger-like processes ; 

 then the region between the processes gradually fills out until 

 a fan-like form is attained. This soon becomes entirely homo- 

 geneous near the periphery and adheres closely to the cover glass. 

 Frequently the fan has a slightly fluted appearance as if not 

 adhering equally well at all points. 



As the mesenchyme cells migrate outward they exert a pull 

 upon the edge of the ectoderm which is drawn outward, not by 

 amoeboid processes of its own, but by amoeboid processes and 

 fan-like expansions of the mesenchyme cells to which it is at- 

 tached. The sheet of ectoderm is thus anchored in all directions 

 as if by minute guy-ropes. When these are pulled more in one 

 direction, folds and wrinkles are formed, which become smoothed 

 out when the pull is equalized. 



