TYPES OF CHORDATE BLASTULAE 357 



As in Amphioxus, the amphibian blastula may be resolved into a two- 

 layered structure composed of a presumptive entodermal or hypoblast layer 

 and an upper, epiblast layer of presumptive epidermal, notochordal, meso- 

 dermal, and neural tissues. Each of these layers, unlike that of Amphioxus, 

 is several cells in thickness. 



3. Mature Blastula in Birds 



Development of the hen's egg proceeds rapidly in the oviduct (fig. 157B-G), 

 and at the time that the egg is laid, the blastodisc (blastula) presents the 

 following cellular conditions: 



( 1 ) a central, cellular blastoderm above the primary blastocoel and 



(2) a more peripheral portion, associated with the yolk material forming 

 the germ-wall tissue (fig. 156G). 



The central blastoderm is free from the yolk substance and is known as 

 the area pellucida, whereas the germ-wall area with its adhering yolk material 

 forms the area opaca (fig. 170). Around its peripheral margin the area 

 pellucida is somewhat thicker, particularly so in that region which will form 

 the posterior end of the future embryo. In the latter area, the pellucid margin 

 may consist of a layer of three or even four cells in thickness (fig. 172A). 

 This thickened posterior portion of the early pellucid area forms the embryonic 

 shield (fig. 170). Anterior to the embryonic shield, the pellucid area is one 

 or two cells in thickness (figs. 171 A; 172B). 



Eventually the pellucid area becomes converted into a two-layered structure 

 with an upper or overlying layer, the primitive ectoderm or epiblast and a 

 lower underlying sheet of cells, the primitive entoderm or hypoblast (figs. 

 171 A; 172A). The space between these two layers forms the true or secondary 

 blastocoel. The cavity below the hypoblast is the primitive archenteric space. 

 At the caudal and lateral edges of the pellucid area, cells from the inner zone 

 of the germ wall appear to contribute to both hypoblast and epiblast. 



The two-layered condition of the avian blastula shown in figure 171 A may 

 be regarded as a secondary or late blastula. At about the time that the sec- 

 ondary blastula is formed (or almost completely formed), the hen's egg is 

 laid, and further development depends upon proper incubational conditions 

 outside the body of the hen. Shortly after the latter incubation period is 

 initiated, the primitive streak begins to make its appearance in the midcaudal 

 region of the blastoderrn, as described in Chapter 9. 



Much controversy has prevailed concerning the method of formation of 

 the entoderm and the two-layered condition in the avian blastoderm. Greatest 

 attention has been given to the origin of the entoderm in the eggs of the 

 pigeon, hen, and duck. The second layer is formed in the pigeon's egg as it 

 passes down the oviduct, in the hen's egg at about the time of laying, and 

 in the duck's egg during the first hours of the external incubation period. The 



