THE ANATOMY OF BIBDS.— OOLOGY. 



231 



two cleavage-cells in place of the one parent-cell. A fun-ow 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 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 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. 

 Til is 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 fi'om 

 the surface of the food-yelk ; a second 

 crystal having grown under the first 

 one. Tlie second adheres to the first, 

 obliterating the original cleavage-cav- 

 ity ; the germ is now obviously two- 



FiG. 112. — Fnrtlier developnieiit of hen's egg; after Ilaeckel: 

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

 fig 111, F, here viewed in profile in section, resting upon n, the 

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

 mass of foofl-yelk. A, morula stage (as before); B, blastula 

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

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

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

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

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

 to Mioct tlie outer results in a cavity '"^ *''® f'leavage-cavity, s; D, the disc-gastrula completed, by 

 . 1 1 <■ 1 11 T-» 7 union of entoderm, i, with exoderm, c, leaving the primitive 



between itsclt and the lood-yelk, I), d. intestinal cavity, d, which is quite similar in appearance to the 

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

 (iriii^iual cleavage-cavity, but it is a very different thing, being the primitive i'ntestinal cavity. 

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

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

 cnidal fnnii which causes it lo be termed a discogastrula. This process of forming a single 

 blastddcniiic 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 ])rimitive 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 

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

 and iB therefore not a blastula proper. 



