BIOLOGICAL ASPECTS OF THE YOLK OF THE HEN'S EGG
RUTH BELLAIRS
Department of Anatomy and Embryology,
University College, London, England
I. Introduction
217
II. The Laid Egg
218
A. The Vitelline Membrane
219
B. The Chemistry of the Yolk
222
C. The Physical Structure of the Yolk
225
III. The Formation of Yolk
231
A. Oogenesis prior to Yolk Formation
232
B. Oogenesis during Yolk Formation
241
C. The Vitelline Membrane and Ovulation
242
IV. The Intracellular Yolk and its Digestion
243
A. The Bate of Disappearance of Yolk
249
V. The Yolk Sac
. '.
250
A. The Origin of the Yolk Sac
250
B. Theories on the Origin of Cells or Nuclei from Yolk
256
C. Further Differentiation of the Yolk Sac
257
D. The Ingestion of Yolk by the Yolk Sac and the Embryo
258
E. The Yolk Sac as a 'Transitory Liver'
_
259
VI. Chemical Changes in the Yolk during Incubation
260
A. Uptake of Water
260
B. Enzyme Activity in the Yolk and Yolk Sac
260
VII. Development of the Embryo without Yolk
263
A. The Explantation Technique
263
£ . Yolk Replacement in situ
265
VIII. Post-Hatching Fate of Yolk and Yolk Sac
265
References
266
I. Introduction
The embryonic life of a domestic chick lasts for 21 days. At the beginning of this period the protoplasm is too small to be seen by the naked
eye; at the end, the newly hatched bird is big enough to hold in the
hand. The raw materials from which this great increase has come are
provided almost entirely by the yolk.
These facts have, of course, been known for centuries and have been
a constant source of fascination to poultry keepers and to scientists.
But it is only within about the last fifty years that we have begun to
understand the intricacies of the problems involved, and perhaps only
217