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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 fowr 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, 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 dise 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 blastodermie membrane and cleav- 
age-cavity are formed, the germ is 4 
ealled a blastula, or germ-vesicle,+ and : 
the process by which the morula be- 
comes a blastula is called blastulation. yw 
Next, from the thickened rim, w, of [i 
the watch-crystal-like blastula a layer 
of large entoderm cells, fig. 112, C, @, : 
separates, and grows toward the centre: a iil a il 
when it gets there, of course the origi- mn ! 
nal cleavage-cavity, s, is shut off from Fic. 112. — Further development of hen’s egg; after Haeckel: 
the surface of the food-yelk a second A, the mulberry mass of cleavage cells, b, same as seen on top in 
; fig. 111, #, here viewed in profile in section, resting upon n, the 
erystal having grown under the first simply-shaded part of the figure, to represent conventionally the 
one. The second adheres to the first, mass of food-yelk. A, morula stage (as before); 8, blastula 
: i ‘ey: stage, the mass of cells, 6, forming the eye aes uplifted from 
obliterating the original cleavage-cav- the food-yelk, leaving the cleavage-cavity, s; w, the thickened 
ity; the gern is now obviously two- rim of the germ-disc; C, the blastula in process of inversion, by 
S Mie 4 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, s; D, the disc-gastrula completed, by 
5 3 ie. 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, s, but morphologically quite different. 
original cleavage-cavity, but it is a very different thing, being the primitive imtestimal 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 eireular dis- ° 
coidal form which causes it to be termed a discogastrula. This process of forming a single 
blastodermie 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 
eavity (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.? 
a 
HH 
ii 
SS 
il 
— 
A 
Hl Il Hut 
WU MULL i 
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 
