226 GENERAL OBNITROLOGY. 



What we have got now is a tread or germ consisting of a circular concavo-convex disc of 

 two layers of hlastodenn, resting hy 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 gastralation, may go on liefore 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 

 nmllerian duct so named, is not so fanciful as some appear to think. The outer of the two 

 blastodermic layers is the ectoderm or epihlast, G or D, e; the inner is the endoderm or /ij/jjo- 

 hlast, i. By multiplication of cells between the two arises the mesoUast. The mesoblaatic 

 layer of cells subsequently splits into two, of which the outer is the somatopleura, or body 

 layer, the inner the splanclmopleura 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- 

 ized in function, — a sort of division-of-labor system in the iufiint colony of cells, by which some 

 are to learn to move, others to digest, others to procreate, others to think and feel, M'ith corre- 

 sponding modifications of form by which are generated the Osteamahce, Myamahm, Neur- 

 amcebce, — the bone-cells, muscle-cells, nerve-cells, and all others of the complex organism 

 which is in a few days to come into being tVoui such simple beginnings. This of course opens 

 up the whole field of embryology, wliich we cannot here enter upon. I will only add, that from 

 the ejjiblast is derived the integument, and its inversions, as tliose of the eye and ear, and the 

 brain and spinal cliord. 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 metamorjihoses 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 

 tliese incubate. "The exception to the rule of incubation is given by the Mcgapodial 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 tlie 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 foi- few birds ; it is known to range from ton days (perhaps less), as in case of 

 the -men, 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 doubk monog- 

 amy. In most or all Ratitce, in the family Phalaropodidce, and some other Limicoliue genera, 

 the male incubates. Most birds attend to their own eggs; many cuckoos (Cueulida) 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 



