1879.] 905 [Minot, 
Of the three axes the perpendicular is the most important, so 
much so that important conclusions may be drawn from the altera- 
tions in the height of the epithelium alone. If the original height be 
determined, then in later stages any departure from that height cor- 
responds to a modification, and if we can safely reason from the 
henogenetic succession to the phylogenetic, then the earliest animal 
with distinct germinal layers, must have consisted of two sets of 
cubicallayers. Either an increase or diminution of the perpendicular 
axis must be regarded as a variation. Let us take the body cavity 
of vertebrates as an example. ‘The peritoneum is at first a layer of 
cubical epithelial cells, as has been shown by many authors!; through- 
out its greater extent the cubical cells are ultimately completely 
flattened, being converted into an endothelium. Along the dorsal 
surface of the body cavity there are four bands, two on each side, 
where the layer retains its primitive character for a longer time; the 
two bands nearest the mesentery form projecting folds, which are 
converted into the sexual organs. The two outer or lateral bands 
form furrows, the first development of the excretory system; the 
differentiation of these organs depends therefore in part on the 
preservation of the primitive form of the cells. So in many other 
cases it will be found that in one part of an epithelium the primitive 
dimensions of the cells are preserved longer than in another part. 
Another point to be considered in this connection, is that changes in 
the height of an epithelium are by no means unimportant, but on the 
contrary they are among the very most important and fundamental 
of all the changes with which the embryologist has to deal. It is on 
this account that the nature of the epithelium should be carefully 
considered in every case. I may incidentally remark that it is for 
this reason that I have emphasized? in my articles on the tracheae of 
insects the fact that in all the species I have examined the external 
epithelium of the air tubes is flat. Chun has stated that he found a 
cubical epithelium, a view which seems to me improbable, for it would 
indicate that in some species an embryonic peculiarity is preserved, 
which I know from direct observations has certainly disappeared 
from closely allied species. 
The nervous system, although arising from an epithelium, develops 
1 IT may mention Goétte, Balfour, Kélliker, Oellacher and others. 
2Minot. Arch. Physiol. norm. pathol., 111, 1. American Naturalist, July, 1877. 
