THE ANATOMY OF BIBBS.— OOLOGY. 



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

 redivision of the nuclei, results 4n f(mr cleavage-cells. Radiating furrows intermediate to the 

 first two bisect the four cells, and 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 shovm 

 in fig. 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 multiphcation 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 m^mbrcme, — the blastodermic mem- 

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

 The cavity between the blastoderm i , 

 and the mass of food-yelk is called the | 

 cleavage cavity, s. At the stage when 

 the blastodermic membrane and cleav- -~r 

 age-cavity are formed, the germ is I 

 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 oflf from Fio. 112. —Further development of hen's egg; after Haeckel : 



the surface of the food-yelk ; a second f tll^ ™»ib^"y •?»' f. de^vage cells, b same as seen on top in 



_ J > flg. iiij F^ iiere viewed in profile in section, resting upon «, the 



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



one. The second adheres to the first, ""^^^ °f f""'l-y«i^. -^. "orula stage (as before); -B, blastula 



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

 obliterating the original oleavage-cav- the food-yelk, leaving the cleavage-cavity, s; w, the thickened 

 ity; the germ is now obviously two- rim of the germ-disc; C, the blastula in processor inversion, by 

 •^ ' ^ ° _ ^ ** which a layer of entoderm-cells, i, growing from periphery to 



layered ; the rising of the inner layer centre, will apply itself to the layer of exoderm-cells, c, obliterat- 

 to meet the outer results in a cavity ■"? "'« cleavage-cavity, s; D, the disc-gastrula completed, by 

 J T. r J Ti Ti J "nion of entoderm, i, with exoderm, e, leaving the primitive 

 between itself and the food-yelk, D, a. intestinal cavity, d, which is quite similar in appearance to the 

 This cavity exactly resembles the cleavage cavity, «, 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 

 blastodennic 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 meroblastie eggs 

 which have a discoidal cleavage and form a discogastrula.^ 



1 Hot to be confounded with the original "germinal vesicle " of the parent-cell,, which long since disappeared 



2 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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