424 



ZOOLOGY 



SECT. 



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

 into two parts, a central, clear area pellucida (Fig. 1086, ar . pi.) and 

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

 ectoderm having below it a somewhat irregular aggregation of 

 cells not yet forming a definite layer. 



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 ventral direction. A primitive 

 knot (p. 353) is absent as a distinct structure, and there is no 

 invagination. In the posterior part of the area pellucida behind 

 the embryonic shield appears a longitudinal opaque band, the 

 primitive streak (pr. st.), 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 



7 pr.sl 



me.s 



FlQ. 1086. Callus bankiva. Two stages! n the development of the blastoderm : diagram- 

 matic, ar. op. area opaca ; ar. pi. area pellucida ; M. head ; med. gr. medullary groove ; 

 mes. mesoderm, indicated by dotted outline and deeper shade ; pr. am. pro-amnion ; pr. st. 

 primitive streak ; -pr. v. protovertebrte. (From Marshall's Embryology, in part after Duval.) 



proliferation 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 

 forwards into the space between the ectoderm and yolk-endoderm, 

 that the foundations of the mesoderm are formed. In the anterior 

 primitive streak-region the primitive knot of the Reptiles is repre- 

 sented by a close union, for a short space, of all three layers. In 

 front the primitive streak becomes free from the ectoderm and 

 unites below with the endoderm : this anterior extremity of the 

 primitive streak is known as the head-process. 



As there is no invagination in Birds in general, there is no primitive 

 endoderm, and the definitive endoderm is formed solely from cells 

 underlying the embryonic shield. The notochord is formed by an 

 axial modification of the endoderm cells along the anterior primitive 

 streak-region and the head-process. In the latter is formed the 



