204 EMBRYOLOGY 



may be present if they are included in the diet of the hen during the period 

 when yolk is formed. 



Upon ovulation the large egg is fertilized as soon as it enters the oviduct. 

 Cleavage and part of gastrulation occur in the oviduct, which provides a 

 suitable temperature (about 103° F.) for development. The developing egg 

 remains in the body of the hen for about 20 hours and is then laid. If the egg 

 cools to room temperature (about 80° F.), development ceases and the 

 gastrula eventually dies. If the temperature is maintained at about 103° F., 

 development continues. While in the body of the hen the egg receives coat- 

 ings of egg albumen secreted by the walls of the oviduct, shell membranes, 

 and a porous calcium carbonate shell from the shell gland. Thus, of the egg 

 that is laid only the "yolk" represents the ovarian egg. The viscous, watery 

 albumen serves to suspend the egg within the shell membranes and maintains 

 an aqueous environment for the developing egg. The shell gives mechanical 

 support and, being porous, allows gases to pass to and from the embryo. Let 

 us return now to the egg in the oviduct just after ovulation. 



Cleavage 



Since the protoplasm of the hen's egg is localized in the form of a small 

 disk, the blastodisk, at one pole of the large egg, and since cell divisions do 

 not extend through the entire egg, the early cleavages merely break up the 

 blastodisk into a large number of cells. The yolk remains intact and non- 

 cellular. Such cleavage is called incomplete, or meroblastic, cleavage. The egg 

 is a telolecithal egg, i.e., yolk concentrated at one pole, protoplasm concen- 

 trated at the other. Thus cleavage of the egg results in a flat sheet of cells, a 

 blastoderm, which floats on top of the yolk (Fig. 119). A cavity, the blasto- 

 coel, develops under the blastoderm. The roof of the blastocoel is composed 

 of cells, whereas the floor is noncellular yolk. The marginal periblast at the 

 edge of the blastoderm contains nuclei which divide and form new cells, so 

 that the edges of the blastoderm spread over the surface of the yolk as 

 indicated by the arrows in Figure 119. During subsequent events, which will 

 be described, the blastoderm increases in size by the addition of cells at its 

 periphery. The marginal periblast soon forms a zone consisting of cells 

 which migrate over the surface of the yolk, and this is termed the zone of 

 overgrowth. Overgrowth continues long after the embryo has formed and 

 ceases only when the entire yolk mass has been incorporated into a sac. 



