320 ANATOMY OF THE DOMESTIC FOWL 



When the ovum is discharged into the ovarian pocket it is surrounded 

 by spermatozoa. 



After the male pronucleus has united with the female pronucleus 

 in the single-celled ovum, there is a cleavage of the cell in the 

 long axis of the egg, making two cells; and then a cleavage at right 

 angles, which progressively continues, makes the mulberry-like 

 mass. The remaining content of the egg consists of food for the 

 development of the embryo. From this mass of cells before the egg 

 is laid the blasioderm is formed. The cells of the blastoderm are 

 differentiated into two layers. The superficial layer is the ectoderm 

 and the lower layer is the entoderm. In the newly laid egg the blasto- 

 derm may be observed. It is about 4 nuUiineters in diameter. 

 It has a transparent central area, the zona peUucida, which is located 

 over the subgerminal region. There is a peripheral, less transparent 

 area called the zona opaca. 



In thie fertile egg, as soon as it is subjected to the proper tempera^ 

 ture, cell multiplication in the blastoderm begins. The first signs 

 of such change are noted in the pellucid area of the blastoderm 

 where embryonal traces appear in the form of the parallel lines 

 called the plica primitivtz, -which diverge to form the cephalic 

 dilatation. At about this time takes place the formation of the 

 myelencephalous columns, in which the blood lakes expand in the 

 surrounding halones and in the tracts along which pass colorless 

 blood particles. These tracts extend from below the cephalic 

 expansion to the peripheral sinuses, as the proto-vertehrce, which 

 begin to appear at the sides of the myelon. The red color is ac- 

 quired by the blood, and the heart by its movements, is made more 

 manifest as the punctum saliens. A distinct membrane, the serous 

 layer, is formed upon the germ and the blastoderm. The cephalic 

 end of the embryo rises from the surface of the blastoderm, and then, 

 ciurving down, sinks into it, forming for itseK a kind of hood of the 

 serous layer. This hood gradually extends from the margin of the 

 fossa over the body, and, naeeting a similar fold formed by the 

 projecting and incurved taU, closes over the germ on the upper side, 

 making a circumscribed cavity, which is the amnion. The progress 

 of differentiation of layers of the blastoderm has meantime, gone on 

 beneath. The serous layer is in part reflected from the vascular 

 and from the mucous layer. The mucous layer is concerned in the 

 formation of the intestinal canal; and beyond this part, which is at 

 first an open groove, the mucous layer expands over the yolk. 



