232 GENERAL OBNITHOLOCxY. 



"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 stiU 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 epiUast, 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 sonmtopleura, 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 marvelhtus process by which 

 the indifferent cells of the blastodermic layers are to become differentiated in form and special- 

 ized 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 corre- 

 sponding modifications of form by which are generated the Osteamcebte, Myamoehee, 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 fi-om 

 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 ofisets, 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 morjjhologist, 

 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 Tnaterial 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 featliers sufficiently developed to enable it to take a sliort 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 tenned double monog- 

 amy. In most or all Eatitce, in the funnily Phalaropodidce, and some other Limicoline genera, 

 the male incubates. Most birds attend to their own eggs; many cuckoos (Cuculidce) 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-layiug process, which does not permit several eggs 



