SEC. IV ANA TOMY OF BIRDS 335 



time of formation of four layers, the cells are all alike, or only differ 

 slightly in size, colour, or consistency. Now, however, ensues that 

 marvellous process by Avhich the indifferent cells of the blastodermic 

 layers are to become differentiated in form and sj^ecialised in function, 

 — a sort of division-of-labour 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, Avitli corresponding modifica- 

 tions of form by which are generated the Osteamceha', Myamoibce, 

 Neuramcehce, — 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 are derived the integument, and its in- 

 versions, as those of the eye and ear, and the brain and spinal 

 cord. 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 pleuroperitoneal 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° Fahr. 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 utilised 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 

 dejwsited vertically in a circle at a certain depth, near the summit, 

 and the chick is developed with the aid of the heat of fermentation. 

 The large size of the egg relates to affording a supply of material 

 sufiicing 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 b}^ the 

 male is termed double monogarmj. In most or all Ratita', in the 



