580
E T I E N N E
W O L F F
regulation of isolated blastomeres were carried out by Seidel (1952) and
Tarkowski (1959a, b) on mouse eggs. Also, some experiments by Pincus
(1939) are in the same category. Pincus was concerned with parthenogenetic activation of rabbit eggs in vitro, followed by their reimplantation in the uterus: in some instances his parthenogenetic eggs developed normally; he recorded that three of them reached the end of their
development and gave rise to normal young.
The experiments of Tarkowski on the fusion of mouse eggs illustrate
the second type of investigation. Nicholas and Hall (1942) had successfully carried out the first experiments on the fusion of two mammalian
eggs. They used two fertilized, non-segmented eggs of rat, from which
the zona pellucida had been removed, and compressed them one against
the other until they adhered closely. The eggs themselves came from
a pigmented type—they were implanted in the uterus of an albino
mother. Only one of these attempts proved successful. The composite
embryo, removed towards the end of gestation, appeared to be quite
normal in make-up with no traces of its dual origin in constitution or
shape. But in size it was much greater than a normal embryo at that
stage—and this seemed to be the only indication that it had originated
from two joined eggs.
However, the two members of such a union might still separate in
the uterus. T o avoid all uncertainty on this point, Tarkowski (1961)
undertook a remarkable series of investigations, on mouse eggs, in
which he combined in vitro culture with reimplantation in the uterus.
Mouse eggs of albino A 2 G type or LAB Grey type or from a crossing of
these two were removed at the eight-cell stage. The zona pellucida was
removed from each and they were placed in pairs in a small drop of
solution surrounded by paraffin oil whose pressure forced the two eggs
one against the other. The fused eggs were cultivated for 24-40 h in
Whitten's medium (see p. 576), made up mainly of Krebs-Ringer
bicarbonate solution, glucose, and beef albumin (V Armour fraction).
After they had attained the blastocyst stage of development, the eggs
were reimplanted in the uterus.
During the period of in vitro development control could be exercised
on whether the two eggs remained fused and whether their cells
collaborated to form one whole. In fact, in most cases, only one embryonic mass and one blastocoel were formed. The volume of the blastocyst
and of the internal mass were double the norm. The eggs which were
cultivated for 24 h were implanted in the oviducts of a LAB Grey
mouse which had had sterile coition on the previous day; the eggs,
cultivated for 40 h, were grafted into the uterus of a mouse after 4
days of pseudogestation. Experiments with the combination of (LAB
Grey) + (LAB Grey x A 2 G ) gave particularly favourable results because
E T I E N N E
W O L F F
regulation of isolated blastomeres were carried out by Seidel (1952) and
Tarkowski (1959a, b) on mouse eggs. Also, some experiments by Pincus
(1939) are in the same category. Pincus was concerned with parthenogenetic activation of rabbit eggs in vitro, followed by their reimplantation in the uterus: in some instances his parthenogenetic eggs developed normally; he recorded that three of them reached the end of their
development and gave rise to normal young.
The experiments of Tarkowski on the fusion of mouse eggs illustrate
the second type of investigation. Nicholas and Hall (1942) had successfully carried out the first experiments on the fusion of two mammalian
eggs. They used two fertilized, non-segmented eggs of rat, from which
the zona pellucida had been removed, and compressed them one against
the other until they adhered closely. The eggs themselves came from
a pigmented type—they were implanted in the uterus of an albino
mother. Only one of these attempts proved successful. The composite
embryo, removed towards the end of gestation, appeared to be quite
normal in make-up with no traces of its dual origin in constitution or
shape. But in size it was much greater than a normal embryo at that
stage—and this seemed to be the only indication that it had originated
from two joined eggs.
However, the two members of such a union might still separate in
the uterus. T o avoid all uncertainty on this point, Tarkowski (1961)
undertook a remarkable series of investigations, on mouse eggs, in
which he combined in vitro culture with reimplantation in the uterus.
Mouse eggs of albino A 2 G type or LAB Grey type or from a crossing of
these two were removed at the eight-cell stage. The zona pellucida was
removed from each and they were placed in pairs in a small drop of
solution surrounded by paraffin oil whose pressure forced the two eggs
one against the other. The fused eggs were cultivated for 24-40 h in
Whitten's medium (see p. 576), made up mainly of Krebs-Ringer
bicarbonate solution, glucose, and beef albumin (V Armour fraction).
After they had attained the blastocyst stage of development, the eggs
were reimplanted in the uterus.
During the period of in vitro development control could be exercised
on whether the two eggs remained fused and whether their cells
collaborated to form one whole. In fact, in most cases, only one embryonic mass and one blastocoel were formed. The volume of the blastocyst
and of the internal mass were double the norm. The eggs which were
cultivated for 24 h were implanted in the oviducts of a LAB Grey
mouse which had had sterile coition on the previous day; the eggs,
cultivated for 40 h, were grafted into the uterus of a mouse after 4
days of pseudogestation. Experiments with the combination of (LAB
Grey) + (LAB Grey x A 2 G ) gave particularly favourable results because