260 CELLS, TISSUES, AND ORGANISMS 



into the cells. Needless to say, confirmation of these extremely inter- 

 esting and important experiments will be awaited with the greatest 

 interest. If they are confirmed, and if they can be extended to other 

 biological systems, they may almost prove the point we made at the 

 beginning of the present paper; namely, that cell differentiation is the 

 consequence of the synthesis of specific proteins, mediated by small 

 RNA-containing particles such as the microsomes or the ribosomes. 



It is of interest in this respect to mention recent observations of 

 Rounds and Flickinger (1958) and Flickinger et al. (1959), who de- 

 tected, with chemical and immunological methods, a definite but quan- 

 titatively small transfer of nucleoproteins from mesoderm to ectoderm. 

 Of special interest are experiments in which Taricha ectoderm was cul- 

 tivated in contact with Rana mesoderm. Serological tests showed the 

 presence of Rana antigens in the Taricha ectoderm, indicating again a 

 passage of nucleoproteins from the mesoderm to the ectoderm. It cer- 

 tainly would be very interesting to follow this process cytochemically 

 with methods using labeled antibodies. 



On the other hand, the work of Grobstein (1955, 1956) has shown 

 that direct contact between inducing and reacting cells is not always 

 required for induction. Working on the induction of tubules in 

 metanephrogenic mesenchyme, he found that the inducing stimulus is 

 not stopped by the interposition of a "millipore" membrane. Such a 

 membrane has large pores, as compared with those of a cellophane 

 membrane which completely stops neural induction (Brachet and 

 Hugon de Scoeux, 1949). Its pores are not large enough to allow the 

 passage of free cells, but they can become filled with long pseudopodia, 

 which apparently never come in direct contact ( Grobstein, 1955, 1956; 

 Grobstein and Dalton, 1957 ) . The active substance, which cannot cross 

 a cellophane membrane, can act at a distance of more than 80 microns 

 ( Grobstein, 1958 ) . For all these reasons Grobstein believes that induc- 

 tion is mediated not through direct contact or diffusion of a small- 

 molecular- weight substance but through the matrix uniting the cells. 



From the embryological viewpoint, the intercellular matrix of 

 Grobstein is a development of Holtfreter's ( 1943 ) surface coat— a ma- 

 terial which is soluble in alkaline media and presents a marked elas- 

 ticity. This material is already present in the cell cortex in the fertilized 

 egg, and it holds the cell together, thus acting as an intercellular ce- 

 ment. The surface coat becomes reinforced, at the time of gastrulation, 

 in the dorsal lip; at the neurula stage it is still further developed in the 

 neural plate. Dissolution of the surface coat by weak alkalis (KCN, for 

 instance) results in separation of the cells that form the embryo. 



Very recent experiments of Gurtis ( 1960 ) show that, as had been 

 deduced on theoretical grounds by Dalcq and Pasteels (1938), this 



