132 
mold, and temporarily fastening the two halves together, a 
form in which wax models can be poured is obtained. 
The study of cell shapes in three dimensions is still in its in- 
fancy; only a few tissues have been investigated—largely, with- 
out doubt, because of the difficulty of making accurate determi- 
nations. In addition to the form of the mature cells, the Stras- 
burgers of the future will unfold, step by step, the changes that 
the cells undergo, from the time of their origin, usually at the 
growing apex, to the time that they are fully differentiated. Isit 
too sanguine to hope that this process will not merely be traced, 
but understood in terms of the mechanics by which it is under- 
lain 
COLUMBIA UNIVERSITY 
NEw YORK 
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