THE 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 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. 111, 4-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-erystal 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. 
ISS Beeatsre 
Tk : J esa oe ian, Cows Seeisroze, iy 
e cavity between the blastoderm ‘o il ih 
and the mass of food-yelk is called the uh I HR 
cleavage cavity, s. At the stage when Ra ee 
the blastodermic membrane and cleav- eaKent a ee 
age-cavity are formed, the germ is fl] | ih i 
) 
called a blastula, or germ-vesicle,! and ; | , 
the process by which the morula be- 
eomes 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, ¢, stu 
a He 
when it gets there, of course the origi- an iM 
nal cleavage-cavity, s, is shut off from Fra. 112. — Further development of hen’s egg; after Haeckel: 
Z Ee 1, the mulberry mass of cleavage cells, 0, same as seen on top in 
the surface of the food y elk ; a second fig. 111, ¥, here viewed in profile in section, resting upon 7, the 
crystal having grown under the first simply-shaded part of the figure, to represent conventionally the 
mass of food-yelk. 4, morula stage (as before); B, blastula 
ne: The second adheres to the first, stage, the mass of cells, 6, forming the blastoderm, uplifted from 
obliterating the original cleavage-cav- the food-yelk, leaving the cleavage-cavity, s; w, the thickened 
Big as 2 * _ rim of the germ-disc; (, the blastula in process of inversion, by 
ity; the Cru: 8 nen obviously two 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, e, obliterat- 
to meet the outer results in a cavity ing the cleavage-cavity, 85 D, the dise-gastrula completed, by 
union of entoderm, i, with exoderm, e, leaving the primitive 
between itself and the food-yelk, D, d. 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 
blastodennie 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.? 
ti = 
1 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 
