GENERAL ZOOLOGY 



Amniotic fold 



Eggshell 



Egg 

 membrane 



Fig. 5.24. ITie embryonic membranes of the chick. A, B, and C, diagrams of 

 longitudinal sections of successive stages during development. (Redrawn v^'ith 

 modifications from T. J. Parker and W. A. Haswell, Textbook of ^oology, copy- 

 right 1921 by Macmillan and Co., Ltd., printed by permission.) 



partial, and the yolk-laden part of the zygote is not divided into cells. 

 Correlated with this fact we find the blastoderm rapidly extending over the 

 surface of the yolk (Fig. 5.20). The cells of the endoderm and its adjacent 

 mesoderm form a yolk sac which eventually encloses the yolk, and numerous 

 blood vessels in this embryonic membrane absorb the yolk as it becomes 

 diffusible and carry it to the developing embryo, which sometimes hatches 

 before all its yolk is used. 



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