1880. | A Sketch of Comparative Embryology. 879 
to special functions. Of course it is not possible to consider here 
in detail the laws of histological differentiation, the more as they 
have never received much attention, for although hundreds of 
published researches elaborately describe the changes in special 
cases, yet the general laws of the progressive development of 
cells have never been seriously discussed, and rarely subject to 
more than incidental treatment. I shall mention only three gen- 
eral principles, which are at once universally applicable and 
readily understood. 1. Structural modifications of epitheliums 
usually affect similarly a whole cluster of cells; or 2. Less fre- 
quently isolated cells only. 3. The mesodermic tissues are for 
the most part in masses (muscles, tendons, fat, etc.) not in layers, 
excepting always the epithelial lining of the mesodermic cavities. 
We have already considered one illustration of the first princi- 
ple, the formation of the central nervous system 
(Fig. 25, #, 2). Other areas are transformed into 
the retinz, the finger nails, etc, Again smaller 
clusters into the lining of glands. Let us consider 
for a moment the peptic glands of the mammalian 
stomach, which are modifications of the simpler 
mucous glands (Fig. 24). The peptic gland is still 
a straight tubular pit running down from the inner 
surface of the stomach, but the cells composing its 
walls are of several kinds—one sort in the neck, a, 
two in the glandular portions, 4 and c, of which 
the darker and more closely granulate cells (“ Be- 
legzellen”’) predominate in 4, but the lighter central 
cells, 4 (“ Hauptzellen”), in c. The central cavity 
of the gland is not shown in 6 and c. The relative 
positions of the two kinds of cells will perhaps be 
better understood by a transverse section, Fig. 27, Bong 
through the lower part, Fig. 26 c, of a cluster ofj. si eae: 
glands, such a section being of course parallel to stomach of guinea 
the inner surface of the stomach. We here have A cnet? 5 
an excellent illustration of what is meant by aa Ariy e sprecseed 
tological differentiation, for the general arrange- Pallen ;” go 
ment of the cells is the same as in Fig. 24, but ae PH 
different parts of the more complex peptic gland diam 
they have assumed distinct forms and functions. 
The differentiation of isolated cells is often very important. In 
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