1880. } A Sketch of Comparative Embryology. 875 
in a cube of three units diameter, zzze units of surface will bound 
twenty-seven units of mass; the proportion in the first cube is 
1:1, in the second 1:3. To maintain the proper proportion in 
the embryo, simple enlargement is insufficient, therefore the 
surface becomes more and more irregular or uneven, being 
thereby multiplied to correspond with the bulk. The irregu- 
larities present distinctive peculiarities characteristic of each 
organ and part, ande may be either large or microscopic. 
They may be conveniently classified under five heads: 1. Pro- 
jections, either large like the limbs of insects and quadrupeds, 
the tentacles of Coelenterates, the branchia of Amphibia, etc., or 
microscopic like the v/i of the intestine.’ 2. Dzlatations of the 
digestive canal and other internal cavities; the stomach is usually 
a dilation. 3. Déverticula, or blind pouches, pushing out from 
one part or another; the lungs of vertebrates, for example, are 
diverticula of the digestive tract. 4. Folds, or ridges either longi- 
tudinal or transverse. A capital illustration is afforded by the 
common grasshopper (Caloptenus); this insect has six large 
diverticula springing from the front end of its stomach, each 
of which is traversed by twelve longitudinal folds, admirably 
shown in transverse sections, Fig.23. 4. Small pits, or invaginations, 
which form glands. They dif- 
fer from diverticula by their 
smaller size, and also in that 
they grow into the mesoderm, 
while the diverticula push the 
mesoderm along with them. A 
section through a couple of such 
pits is shown in Fig. 24, which 
represents “mucous glands” 
from the stomach of a kanga- 
roo. It will be noticed that the 
cells at the bottom of the pit 
are larger than those nearer its 
mouth, so that the lower g/andular portion is already marked off 
from the upper part or duct. Of pits, or glands as they are prop- 
| To see the wi//i, of which students usually have a very imperfect conception, it 
is only necessary to take a short piece of small intestine of a common mammal (dog 
or rat), slit it lengthwise, spread it out, wash it and examine with a lens. Although 
the inner surface of the intestine would be very small if it were smooth, yet in 
reality it is very great, being increased by the countless villi and glands. 
connective tissue. X about 40 di iam, 
