Minot.} 204 [April 2, 
It was not, however, till recently, that the metamorphosis of the em- 
bryo was followed by F. E. Schulzet, who discovered that the 
apparent entoderm becomes external in the adult, the apparent 
ectoderm internal. This very interesting discovery obliges us to 
hesitate before accepting the distinction drawn between the two germ 
layers, as given above. Or else we must suppose, either that the dis- 
tinction is true of only the higher Metazoa, or else that it is true of 
all including the sponges. The latter alternative is the more difficult 
to accept, because it involves the conclusion that the apparent ecto- 
derm of sponges is the equivalent of the entoderm of other animals. 
The difficulty is very serious, the more so as Schulze is so careful and 
skillful an observer that there is no ground to suspect an error of 
observation. 
The further differentiation of two epithelial germ layers depends on 
(1) changes in the proportion of the three axes mentioned above; (2) 
the formation of folds; and (3) differentiation of the structure of the 
single cells. I shall speak of each of these processes briefly. 
3. The two horizontal axes usually remain approximately equal to 
one another in length while the perpendicular axis varies independently 
and to a much greater absolute extent. I have already said that the 
original cells are nearly equiaxial, a statement that every histologist 
and embryologist can readily confirm. In the adult animal, how- 
ever, the ectoderm varies very much in height. The epidermal cells 
are low and broad in the Medusae but very high and narrow in 
Lucernaria, or in Polia, while in Lumbricus they are moderately 
high. In all vertebrates, except of course Amphioxus, the multipli- 
cation of cell-layers in the epidermis introduces a distorting element. 
So also in the entoderm, the epithelium is at first of uniform height, 
but as development progresses some of the cells become higher and 
narrower than at first, or, as in the gastric glands of vertebrates, the 
cells at the base of involutions become not only higher but broader. 
It is not, however, necessary to enter into details, which would only 
lead to the repetition of familiar facts — enough has been said to show 
that the perpendicular axis is the most variable and independent, 
while every one who has examined surface views of epitheliums from 
many organs and species, will recall that although the outlines vary 
very much, yet the two horizontal diameters are, though not equal, 
at least dimensions of the same order, and so nearly alike, that the 
familiar designation of cylinder-epithelium is quite justifiable. 
1 Zeitschr. f. wiss. Zool., xxxi, (1878). 
