266
RUTH BELLAIRS
been absorbed by the wall of the yolk sac (Jordonov and AnastossovaKristen, 1963). I t has, however, been shown by Fritz (1961) that when
the yolk sac stalk is ligatured, résorption is unaffected.
The results of histological studies suggest moreover that the yolk sac
phagocytoses some at least of the residual yolk (e.g. Candiollo, 1960)
although it is possible that some of this phagocytic activity is associated
with the breakdown of the yolk sac itself (Romanoff, 1960; Fritz, 1961 ;
Jordonov and Anastossova-Kristeva, 1963). The phagocytosed yolk
probably leaves the yolk sac by means of the blood vessels as it does in
the embryo. Fritz (1961) showed that if these vessels were ligatured in
the young chick yolk résorption was markedly reduced.
The histological appearance varies from one individual to another,
apparently depending on the amount of residual yolk remaining in it
(Romanoff, 1960; Fritz, 1961). Although the organ has usually disappeared by about 4 weeks, remnants have been seen in healthy male
birds as much as 2 years old (Urban, 1955). Generally, it is believed that
starvation or modification of the diet make little difference to yolk
résorption (Romanoff, 1960; Urban, 1955). However, Crowley et al.
(1960) found that when newly hatched chicks were kept on a special
diet deficient in phosphorus they tended to utilize more phosphorus
from their original yolk stores.
Several authors have described large bodies that appear in the yolk
toward the time of hatching (e.g. Charbonel-Salle and Phisalix, 1886).
Fritz (1961) has shown that these are calcareous.
Although the yolk sac usually disappears, most of the yolk stalk
remains and even elongates, being then known as Meckel's diverticulum.
I t is homologous with the structure of that name that occurs as a congenital anomaly in man. According to Kar (1947) it persists in the
domestic fowl as a finger-shaped projection in the duodenum.
Acknowledgements
I am most grateful to Mrs. J. Astafiev for her great care in preparing
the drawings, and to Miss J. Spillman for her skill in printing the electron
micrographs. I am also deeply indebted to Dr. Elizabeth Deuchar, Miss
M. Greenfield, Dr. J. Williams and Professor J. Z. Young, who have
each read considerable parts of the manuscript and offered helpful
criticisms. I should also like to thank Dr. A. P. Mathias who most kindly
centrifuged samples of yolk for me, and Mrs. Cynthia Jensen, University
of Minnesota, for allowing me to quote her unpublished work.
References
Abe, T., Kaneko, T. and Hosada, T. (1959). Bull. nat. Inst, agric. Sei., Tokyo Ser. G,
18, 229. (In Japanese. See Biol. Abst. (1961)1332.)
RUTH BELLAIRS
been absorbed by the wall of the yolk sac (Jordonov and AnastossovaKristen, 1963). I t has, however, been shown by Fritz (1961) that when
the yolk sac stalk is ligatured, résorption is unaffected.
The results of histological studies suggest moreover that the yolk sac
phagocytoses some at least of the residual yolk (e.g. Candiollo, 1960)
although it is possible that some of this phagocytic activity is associated
with the breakdown of the yolk sac itself (Romanoff, 1960; Fritz, 1961 ;
Jordonov and Anastossova-Kristeva, 1963). The phagocytosed yolk
probably leaves the yolk sac by means of the blood vessels as it does in
the embryo. Fritz (1961) showed that if these vessels were ligatured in
the young chick yolk résorption was markedly reduced.
The histological appearance varies from one individual to another,
apparently depending on the amount of residual yolk remaining in it
(Romanoff, 1960; Fritz, 1961). Although the organ has usually disappeared by about 4 weeks, remnants have been seen in healthy male
birds as much as 2 years old (Urban, 1955). Generally, it is believed that
starvation or modification of the diet make little difference to yolk
résorption (Romanoff, 1960; Urban, 1955). However, Crowley et al.
(1960) found that when newly hatched chicks were kept on a special
diet deficient in phosphorus they tended to utilize more phosphorus
from their original yolk stores.
Several authors have described large bodies that appear in the yolk
toward the time of hatching (e.g. Charbonel-Salle and Phisalix, 1886).
Fritz (1961) has shown that these are calcareous.
Although the yolk sac usually disappears, most of the yolk stalk
remains and even elongates, being then known as Meckel's diverticulum.
I t is homologous with the structure of that name that occurs as a congenital anomaly in man. According to Kar (1947) it persists in the
domestic fowl as a finger-shaped projection in the duodenum.
Acknowledgements
I am most grateful to Mrs. J. Astafiev for her great care in preparing
the drawings, and to Miss J. Spillman for her skill in printing the electron
micrographs. I am also deeply indebted to Dr. Elizabeth Deuchar, Miss
M. Greenfield, Dr. J. Williams and Professor J. Z. Young, who have
each read considerable parts of the manuscript and offered helpful
criticisms. I should also like to thank Dr. A. P. Mathias who most kindly
centrifuged samples of yolk for me, and Mrs. Cynthia Jensen, University
of Minnesota, for allowing me to quote her unpublished work.
References
Abe, T., Kaneko, T. and Hosada, T. (1959). Bull. nat. Inst, agric. Sei., Tokyo Ser. G,
18, 229. (In Japanese. See Biol. Abst. (1961)1332.)