AA'A rOMY OF BIRDS 333 



the parent-cell divides into two ; each attracts its half of the yelk ; 

 the halves furrow apart and there are now iico 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. Radiat- 

 ing furrows intermediate to the first two bisect the four cells, and 

 would render eight cells, Avere 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 jjarticular kind of cleavage, by radiat- 

 ing 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 moment consists increases hy multiplication of 

 cells, and the disc is lifted a little away from the mass of yellow 

 food-yelk upon which it rests, like a w^atch-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 

 membrane, or blastoderm (Fig. 113, 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 cleavage-cavity 

 are formed, the germ is called a Uastula, or germ-vesicle,''- and the 

 process by which the morula becomes a blastula is called blasfula- 

 tion. Next, from the thickened rim, vj, of the watch-crystal-like 

 blastula a layer of large endoderm cells (Fig. 112, C, i) separates, and 

 groAA"^ toward the centre : when it gets there, of course the original 

 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-cavity ; the 

 germ is now obviously ttvo-layered ; the rising of the inner layer to 

 meet the outer results in a cavity between itself and the food-yelk, 

 D, 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 gastridatlon. 

 The gastrula of a bird has the circular discoidal form which causes 

 it to be termed a discogastnda. This process of forming a single 

 blastodermic layer, with a cleavage-cavity (blastula, or true germ- 

 vesicle), then two blastodermic layers, with obliteration of the 

 cleavage -cavity and substitution of a primitive intestinal cavity 



■^ Not to be confounded with the original " germinal vesicle " of the parent-cell, 

 which long since disappeared. 



