TRE ANATOMY OF BIRDS.— OOLOGY. 



225 



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

 redivision of the nuclei, results in four 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 eacli, 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, iu 

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

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

 in fig. Ill, A-F. 



The morula or mulberry-massed germ 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, lo, 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 

 the surface (jf the food-yelk ; a second 

 crystal having grown under the first 

 line. The second adheres to the first, 

 obliterating the original cleavage-cav- 

 ity ; the genu is now obviously two- 

 layered ; the rising of the inner layer 

 to meet the outer results in a cavity 

 lietween itself and the food-yelk, I), d. 

 This cavity exactly resembles the 

 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, M-ith obliteration of the cleavage-cavity and substitution fif 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 Tesicle " of the parent-cell, which long since ilisappeiired. 

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

 and is therefore not a blastula proper. 



Fig. 112. — Farther development of hen's egg; after llaeckel: 

 A, the mulberry mass of cleavage cells, b, same as seen on top in 

 fig- 111, F, liere viewed in proiile in section, resting upon n, the 

 simply-shaded part of the figure, to represent conventionally the 

 mass of food-yelk. A, morula stage (as before),- Jl. blastula 

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

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

 rim of the germ-disc; C, the blastula in process of inversion, by 

 which a layer of entoderm-cells, i, growing from periphery to 

 centre, will apply itself to the layer of exoderm-cells, e, obliterat- 

 ing tlie cleavage-cavity, .?; D, the disc-gastrula completed, by 

 union of entoderm, i, with exoderm, e, leaving the primitive 

 intestinal cavity, d, which is quite similar in appearance to the 

 cleavage cavity, s, but morphologically quite different. 



