THE ANATOMY OF BIRDS. — OOLOGY. 



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two cleavage-cells in place of the one parent-cell. A fiirrow at right angles to the first, and 

 redivisiou of the nuclei, results in fou/r cleavage-cells. Radiating furrows intermediate to the 

 first two hisect the four cells, aud would render eight cells, were not these simultaneously 

 doubled by a circular furrow which cleaves each, with the result of sixteen cleavage-cells. So 

 the subdivision goes on until the parent-cell becomes a mass of cells. This particular kind of 

 cleavage, by radiating and concentric furrowing, is called discoidal, and the resulting heap of 

 little cells assumes the figure of a thin, flat, circular disc. Segmentation of the vitellus, in 

 whatever manner it may go on, results in a mulberry-like mass of cleavage-cells ; and the 

 original cytula has become what is called a morula. This process and result are clearly shown 

 in tig. Ill, A-F. 



The morula or mulberry-massed genn of which the "tread" of a bird's egg at this mo- 

 ment consists increases by multiplication of cells, and the disc is lifted a little away from the 

 mass of yellow food-yelk upon which it rests, like a watch-crystal from the face of a watch. 

 This disposition of the greatly multiplied cells in a layer and their coherence forms of course 

 a membrane, — the blastodermic mem- 

 brane, or blastoderm, fig. 112, B, b. 

 The cavity between the blastoderm 

 and the mass of food- yelk is called the 

 cleavage cavity, s. At the stage when 

 the blastodermic membrane and cleav- 

 age-cavity are formed, the germ is 

 called a blastula, or germ-vesicle,^ and 

 the process by which the morula be- 

 comes a blastula is called blastulation. 

 Next, from the thickened rim, w, of 

 the watch-crystal-like blastula a layer 

 of large entoderm cells, fig. 112, C, i, 

 separates, and grows toward the centre : 

 when it gets there, of course the origi- 

 nal cleavage-cavity, S, is shut off from Fig. 112.— Further development of hen's egg; after Haeckel : 

 the surface of the food-yelk ; a second f ' "?, ^lulberry mass of cleavage cells, h, same as seen on top in 

 . '' ng- Hi, -f, here viewed in profile in section, resting upon n, the 

 crystal having grown under the first simply-shaded part of the figure, to represent conventionally the 

 one. The second adheres to the first, '"^''^ ""^ food-yelk, a, morula stage (as before); B, blastula 

 . . stage, the mass of cells, 6, forming the blastoderm, uplifted from 

 obhteratmg the origmal cleavage-cav- the food-yelk, leaving the cleavage-cavity, s; w, the thickened 

 ity: the germ is now obviously two- "m of the germ-disc; C, the blastula in process of inversion, by 

 J . . p 1 . 1 which a layer of entoderm-cells, i, growing from periphery to 

 layered; the rismg ot the mner layer centre, will apply itself to the layer of exoderm-cells, e, obliterat- 

 to meet the outer results in a cavity *"^ *''® deavage-cavity, s; n, the disc-gastrula completed, by 

 , -i ir 1 iu f ] 11 T-> J union of entoderm, i, with exoderm, e, leaving the primitive 

 between itselt and the tood-yelk, U, d. intestinal cavity, d, which is quite similar in appearance to the 

 Tills cavity exactly resembles the cleavage cavity, s, but morphologically quite different, 

 original cleavage-cavity, but it is a very different thing, being the primitive intestinal cavity. 

 The blastula, or germ-vesicle, has become converted into a gastrula, by the invaginating 

 process just described, known as gastrulation. The gastrula of a bird has the circular dis- 

 coidal form which causes it to be termed a discogastrula. This process of forming a single 

 blastodermic layer, with a cleavage-cavity (blastula, or true germ-vesicle), then two blasto- 

 dermic layers, with obliteration of the cleavage-cavity and substitution of a primitive intestinal 

 cavity (gastrula), is common to all animals which consist of more than single cells, under vari- 

 ous modifications and disguises ; the process described is that occurring in meroblastic eggs 

 which have a discoidal cleavage and form a discogastrula.^ 



' Not to be confounded with the original "germinal vesicle " of the parent-cell, which long since disappeared 

 - The so-called " germ- vesicle" of the holoblastic mammalian egg is subsequent to gastrulation, not prior 

 and is therefore not a blastula proper. 



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