xiir 



PHYLUM CHORDATA 



437 



Segmentation takes place during the passage of the egg down tlie 

 oviduct, and results, as in Reptiles, in the formation of a blasto- 

 derm (Fig. 1074, bl.) occupying a small area on the upper pole of 

 the yolk. After the egg is laid, the process of development is 

 arrested unless the temperature is kept up to about 37° to 40° C. : 

 this is usually done by the heat of the body of the parent Birds, 

 one or both of which sit upon, or incubate, the eggs until the 

 young are hatched ; but in the Australian mound-makers {Mega- 

 podius) the eggs are buried in heaps of decaying vegetable matter, 

 the decomposition of which generates the necessary heat. 



In the newly-laid egg the blastoderm is divisible, as in Reptiles, 

 into two parts, a central, clear area pellucida (Fig. 1075, ar. pi.) 

 and a peripheral area opaca (ar. op.), and is formed of a superficial 



Acl 



pr.%f 



-pr.sl 



Fio. 1075.— Gallas bankiva. Two stages in the development oftheblsistoderm : diagrammatie. 

 ar. op. area opaca ; ar. pi. area pellucida ; hd. head ; med. (jr. medullary groove ; mes. mesoderm, 

 indicated by dotted outline and deeper shade ; })r. am. pro-amnion ; pr. st. primitive streak ; 

 2>r. V. protovertebrae. (Fi'om Marshall's Emhi-yology, in part after Duval.) 



ectoderm having below it a somewhat irregular aggregation of 

 yolk-endoderm cells. 



On the surface of the area pellucida, as in the Reptiles, appears 

 an embryonic shield, the formation of which is due to the elonga- 

 tion of the ectoderm cells in a vertical direction. A primitive 

 knot (p. 364) is not recognisable, and in most Birds there is no 

 invagination. In the posterior part of the area pellucida behind 

 the embryonic shield appears a longitudinal opaque band, the 

 primitive streak, and along the middle of this is formed a groove, 

 the primitive groove. The latter represents the blastopore of the 

 Reptiles, and there is no archenteric cavity. It is by active pro- 

 liferation of cells along the course of the primitive streak, which 

 represents the coalescent lips of the blastopore, leading to the 

 formation of masses of new cells that grow out laterally and 



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