99 ANIMAL PHYSIOLOGY. 
may be made up in part by the exhibition to him of a set of 
specimens permanently mounted or even by plates, nothing can, 
in our opinion, take the place of the examination of eggs as we 
have suggested. It prepares for the study of the development 
of the mammal, and exhibits the membranes in a simplicity, 
freshness, and beauty which impart a knowledge that only 
such direct contact with nature can supply. To proceed with 
great simplicity and very little apparatus, one requires but a 
forceps, a glass dish or two, a couple of watch-glasses, or a 
broad section-lifter (even a case-knife will answer), some water, 
containing just enough salt to be tasted, rendered lukewarm 
(blood-heat). 
Holding the egg longitudinally, crack it across the center 
transversely, gently: and carefully pick away the shell and its 
membranes, when the blastoderm may be seen floating upward, 
as it always does. It should be well examined in position, 
using a hand lens, though this is not essential to getting a fair 
knowledge; in fact, if the examination goes no further than 
the naked-eye appearances of a dozen eggs, selecting one every 
twenty-four hours during incubation, when opened and the 
shell and membranes well cleared away, such a knowledge will 
be supplied as can be obtained from no books or lectures how- 
ever good. It will be, of course, understood that the student 
approaches these examinations with some ideas gained from 
plates and previous reading. The latter will furnish a sort of 
biological pabulum on which he may feed till he can furnish 
for himself a more natural and therefore more healthful one. 
While these remarks apply with a certain degree of force to all 
the departments of physiology, they are of special importance 
to aid the constructive faculty in building up correct notions 
of the successive rapid transformations that occur in the de- 
velopment of a bird or mammal. 
Fig. 103 shows the embryo of the bird at a very early 
period, when already, however, some of the main outlines of 
structure are marked out. Development in the fowl is so rapid 
that a few days suffice to outline all the principal organs of 
the body. In the mammal the process is slower, but in the 
main takes place in the same fashion. 
As the result of long and patient observation, it is now set- 
tled that all the parts of the most complicated organism arise 
from the three-layered blastoderm previously figured; every 
part may be traced back as arising in one or other of these lay- 
ers of cells—the epiblast, mesoblast, or hypoblast. It frequently 
