226 
GENERAL ORNITHOLOGY. 
What we have got now is a tread or germ consisting of a circular concavo-convex disc of 
two layers of blastoderm, resting by its rim upon the great yellow ball of food-yelk, from which 
it is separated by a cavity, as a watch-crystal from its face. All these changes, up to comple- 
tion of gastrulation, may go on before the egg is laid, the tread of a perfectly fresh egg being 
already a multicellular discogastrula. Since the earlier stages of the embryo (cytula, morula, 
blastula, and gastrula) are actually accomplished while the egg is still in the body of the parent, 
the analogy of the oviduct to uterus, etc., as well as its strict homology to the parts of a 
miillerian duct so named, is not so fanciful as some appear to think. The outer of the two 
blastodermic layers is the ectoderm or epiblast, C or D, e; the inner is the endoderm or hypo- 
blast, i. By multiplication of cells between the two arises the mesoblast. The mesoblastic 
layer of cells subsequently splits into two, of which the outer is the somatopleura, or body 
layer, the inner the splanchnopleura or visceral layer. The two-layered germ has then become 
four-layered. Up to the time of formation of four layers, the cells are all alike, or only differ 
slightly in size, color, or consistency. Now, however, ensues that marvellous process by which 
the indifferent cells of the blastodermic layers are to become differentiated in form and special- 
ised in function, — a sort of division-of-labor system in the infant colony of cells, by which some 
are to learn to move, others to digest, others to procreate, others to think and feel, with coiTe- 
sponding modifications of form by which are generated the Osteamoebce, Myamoebce, Neur- 
amoebce, — the bone-cells, muscle-cells, nerve-cells, and all others of the complex organism 
which is in a few days to come into being from such simple beginnings. This of course opens 
up the whole field of embryology, which we cannot here enter upon. I will only add, that from 
the epiblast is derived the integument, and its inversions, as those of the eye and ear, and the 
brain and spinal chord. From the hypoblast is derived the lining of the alimentary canal and of 
its annexes and offsets, as liver, lungs, etc. The rest of the embryo comes from the mesoblast, 
and most of it from the somatopleural layer. The fissure between the two layers of the 
mesoblast becomes the great pleuro-peritoneal cavity. 
In explaining the early embryo, I have closely followed the great German morphologist, 
Haeckel ; and the illustrations are from the same high source. 
Incubation. — To induce the wonderful metamorphoses just hinted at, it is only necessary 
to keep a bird's egg at a pretty even temperature of about 100° F. Nearly all birds secure 
this result by the process of incubation. In many cases the sun's rays relieve the parent of 
some part of the duty. In a few, the heat evolved from vegetable ferment or decomposition is 
utilized for the same purpose. This seems to be the case to some extent with grebes ; but 
these incubate. ''The exception to the rule of incubation is given by the Megapodial birds 
of the Australasian Islands. A huge mound of decaying vegetable matter is raised ; the eggs 
are deposited vertically in a circle at a certain depth, near the summit, and the chick is devel- 
oped with the aid of the heat of fermentation. The large size of the egg relates to affording 
a supply of material sufficing for an unusually advanced state of development of the chick at 
exclusion ; whereby it has strength to force its way to the surface of the hatching-mound, 
with wings and feathers sufficiently developed to enable it to take a short flight to the nearest 
branch of an overshadowing tree" (Owen). The period of incubation has been ascertained 
with precision for few birds; it is known to range from ten days (perhaps less), as in case of 
the wren, to fifty or sixty for the ostrich. The female is usually the sitter. Frequently both 
sexes incubate in turn ; such unnatural care for the young by the male is termed double monog- 
amy. In most or all RatitcE, in the family Phalaropodidce, and some other Limicoline genera, 
the male incubates. Most birds attend to their oAvn eggs; many cuckoos {Cucididce) and the 
species of Molothrus, are parasitical, laying in the nests of other birds, which are thus forced to 
become foster-parents of alien offspring, generally to the destruction of their own. This seems 
to result from some peculiarity of the egg-laying process, which does not permit several eggs 
