THE ANATOMY OF BIRDS.— OOLOGY. 
225 
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 'mfour 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. 
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 
the surface of the food- yelk ; a second 
crystal having grown under the first 
one. The second adheres to the first, 
obliterating the original cleavage-cav- 
ity ; the germ is now obviously two- 
layered ; the rising of the inner layer 
to meet the outer results in a cavity 
between itself and the food- yelk, T>, d. 
Tliis 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 invagiuating 
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 occumng 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 is therefore not a blastula proper. 
15 
Fig. 112. — Further development of hen's egg; after Ilaeckel : 
A, the mulberry mass of cleavage cells, b, same as seen on top in 
fig. Ill, F, here viewed in profile 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); B, 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 peripliery to 
centre, will apply itself to the layer of exoderm-cells, e, obliterat- 
ing the cleavage-cavity, s; 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. 
