15. E M B R Y O G E N E S I S in vitro
565
become established between their vascular areas. A fragment of blastoderm which does not comprise the heart acts like a graft with regard
to the whole embryo with which it is associated. Branches of the extraembryonic system of the principal embryo irrigate the associated
fragment and unite it with their own vessels.
6. The Problem Relating to the Origin and Transport of Primary Gonocytes
The above method allowed Simon (1960) to make an important
contribution to the problem of the origin and migration of primary
gonocytes. Following Swift (1914), several authors have agreed that, at
the primitive streak stage, the primary gonocytes of the embryo of chick
are localized in a crescentic region on the borders between the embryonic area and the extra-embryonic area. Simon has proved this decisively by excising the "genital crescent" between the head process and
ten somites stages. The embryo, examined post mortem on the fourth day,
appeared to have no germ cells whatsoever in its genital rudiments,
although these had differentiated into gonads.
If one associates two blastoderms together, one of which has been
deprived of its genital crescent, both embryos then contain gonocytes
in their developing gonads providing that vascular anastomoses have
formed between them. If such formations have not occurred then the
embryo deprived of its genital crescent remains sterile.
Similarly, if one grafts onto a whole blastoderm the posterior region
of another blastoderm, the gonocytes will colonize not only the gonads
of the host but also those of the graft.
Experiments on heteroplastic parabiosis between embryos of chick
and of duck have yielded similar results. An embryo of chick deprived
of its germinal crescent can incorporate into its gonads germinal cells
of the duck. Conversely, the genital rudiments of a duck embryo
deprived of its germinal crescent can become colonized by the gonocytes of the chick embryo.
Simon's experiments verify in very graphic fashion the fact that the
primary gonocytes are localized in a crescent situated in the anterior
part of the area pellucida in birds. They show further that the gonocytes are transported by the circulatory system from their original
position towards the developing gonads.
C . T H E
N U T R I T I V E N E E D S
A N D
T H E M E T A B O L I C A C T I V I T I E S
O F B L A S T O D E R M S I N
C U L T U R E
1. Minimal Nutritive Needs of Embryos
Spratt (1948) has tried to substitute synthetic, chemically defined culture media for natural media based on albumin and egg yolk. First of
565
become established between their vascular areas. A fragment of blastoderm which does not comprise the heart acts like a graft with regard
to the whole embryo with which it is associated. Branches of the extraembryonic system of the principal embryo irrigate the associated
fragment and unite it with their own vessels.
6. The Problem Relating to the Origin and Transport of Primary Gonocytes
The above method allowed Simon (1960) to make an important
contribution to the problem of the origin and migration of primary
gonocytes. Following Swift (1914), several authors have agreed that, at
the primitive streak stage, the primary gonocytes of the embryo of chick
are localized in a crescentic region on the borders between the embryonic area and the extra-embryonic area. Simon has proved this decisively by excising the "genital crescent" between the head process and
ten somites stages. The embryo, examined post mortem on the fourth day,
appeared to have no germ cells whatsoever in its genital rudiments,
although these had differentiated into gonads.
If one associates two blastoderms together, one of which has been
deprived of its genital crescent, both embryos then contain gonocytes
in their developing gonads providing that vascular anastomoses have
formed between them. If such formations have not occurred then the
embryo deprived of its genital crescent remains sterile.
Similarly, if one grafts onto a whole blastoderm the posterior region
of another blastoderm, the gonocytes will colonize not only the gonads
of the host but also those of the graft.
Experiments on heteroplastic parabiosis between embryos of chick
and of duck have yielded similar results. An embryo of chick deprived
of its germinal crescent can incorporate into its gonads germinal cells
of the duck. Conversely, the genital rudiments of a duck embryo
deprived of its germinal crescent can become colonized by the gonocytes of the chick embryo.
Simon's experiments verify in very graphic fashion the fact that the
primary gonocytes are localized in a crescent situated in the anterior
part of the area pellucida in birds. They show further that the gonocytes are transported by the circulatory system from their original
position towards the developing gonads.
C . T H E
N U T R I T I V E N E E D S
A N D
T H E M E T A B O L I C A C T I V I T I E S
O F B L A S T O D E R M S I N
C U L T U R E
1. Minimal Nutritive Needs of Embryos
Spratt (1948) has tried to substitute synthetic, chemically defined culture media for natural media based on albumin and egg yolk. First of