1879.] 201 (Minot. 
Let us assume that the cells are all spherical, and further that the 
parent cells divide in two — then the division of each cell can occur 
only in one single plane. Even if the number of cells produced at 
one time is greater than two, still as long as they are not more than 
six in number they can arrange themselves in one plane around the 
parent cell. In the majority of cases the cells are not spherical nor 
even spheroidal, but usually compressed, so that a greater number 
than six cells could arrange themselves in one plane in contact with 
oneanother. If the cells instead of remaining in contact with one 
another, as is usual, could separate, an indefinite enlargement in one 
plane could occur. We say therefore: The multiplication of every cell 
occurs in one plane only; if the planes for all the cells of one body were 
parallel, then the growth of the body would be confined to one plane; 
since however the planes of division for the neighbouring cells are 
not parallel but vary towards all three dimensions of space, therefore 
the body grows in all directions. It now becomes evident that if the 
proportion of the cells dividing in each one of the three dimensions 
remains nearly the same throughout life, then determining any one 
dimension of the body must give an approximately accurate curve of 
growth —a conclusion of practical value rather than of theoretic 
importance. | 
The relation of the planes of divisions of the single cells to one 
another is, as I may incidentally remark before concluding, an ex- 
tremely important factor in the processes of histological differentia- 
tion, a subject which I hope shortly to discuss elsewhere. Thus in 
all epitheliums, composed of only a single layer of cells, the plane of 
division must be parallel to the surface of the epithelium. In the 
early stages of embryonic life, for instance in the epiblast, which 
really is an epithelium, the division of the cells has, especially in in- 
vertebrates, been again and again seen to occur in a plane parallel 
to the surface of the layer. To give one more example, in nerves 
the plane of division must be parallel with the fibres. I think there 
can be no question that for every tissue there is a special and char- 
acteristic relation of the planes of division of the single cells to one 
another. ‘The further discussion of these relations must be foregone 
as aside from the objects of this article. 
It remains for me to express my thanks to Dr. H. P. Bowditch, and 
to Prof. Chas. R. Cross for their valuable hints and suggestions. 
= SS 
FSS. SSS, 
