XIII 



PHYLUM CHORDATA 



409 



as large as a Swan's or an Albatross's, the Kiwi itself being no 

 larger than a Barn-door Fowl. 



Segmentation takes place during the passage of the egg down the 

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

 derm (Fig. 1012, bl.) occupying a small area at one pole of the yolk. 

 After the egg is laid, the process of development is arrested unless 

 the temperature is kept up to about 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 (Megapodius) 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 into two parts, 

 a central, clear area pellucida (Fig. 1013, ar. pi.) and a peripheral 



cir.pl 



pr.sl 



FIG. 1013. Gallus bankiva. Two stages in the development of the blastoderm : diagrammatic. 

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

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

 /'/. t-. proto-vertebrse. (From Marshall's Embryology.) 



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

 having below it a somewhat irregular aggregation of lower-layer 

 cells, which gradually become differentiated into mesoderm and 

 endoderm. 



At the posterior end of the blastoderm a delicate, longitudinal, 

 grooved mark, the primitive streak (pr. st.) makes its appearance. 

 Like the similarly named structure in the Frog, it represents the 

 blastopore, but no invagination takes place beyond a solid ingrowth 

 of ectoderm, and the enteric cavity is formed entirely by the folding 

 in of the ventral walls of the embryo. 



Immediately in front of the primitive streak the medullary 

 groove (med. gr.) appears, and the medullary folds which bound it 

 on the right and left diverge posteriorly, so as to embrace the 

 anterior end of the primitive streak, in just the same way as they 



