58
J. C L A V E R T
thoroughly studied, the determination of the plane of bilateral symmetry proceeds from an active reaction of the egg (a real morphogenetic movement) which is directed by an external factor. The same
directive factors are always found in Vertebrata (spermatozoon, rotation of orientation, rotation of the egg with its membranes, compression, position of the germ in space). As the case may be, one or
another of these factors normally acts, but they are all able to orientate
experimentally the plane of symmetry in Rana fusca. The reaction of
the egg leads to a visible formation which indicates precociously the
position of the plane of symmetry. Experimentally a plane of bilateral
symmetry can be imposed on the egg, but the egg itself fixes it by a
specific reaction.
Two types of eggs can be distinguished from a temporal point of view:
(1) the egg in which symmetrization starts early (with total segmentation, i.e., Amphibia, Chondrostei) where the plane of symmetry is
determined before segmentation starts.
(2) the egg in which symmetrization is retarded (with discoidal segmentation, i.e., birds, selachii, teleostei), where determination occurs in a
germ partially or totally segmented.
It seems evident that egg structure is important in the distinction of
these two types of eggs whose behaviour is so entirely different. The answer
is to be found in the distribution of the yolk and not in amount of yolk.
This study of the determination of the plane of bilateral symmetry
permits us to eliminate the pre-formation of symmetry, as Spemann
thought. This is proved by the fact that in all types of eggs the plane of
symmetry can be determined experimentally according to a preselected meridian. The equipotentiality of the different areas of the egg
distributed around the axis of polarity (animal-vegetal pole) is a netv
notion brought forth by this study.
Each meridian can form a plane of symmetry and become the
cephalo-caudal axis. Lastly, the position of the organizing centre is
fixed when the plane of bilateral symmetry is determined. This reaction
of the egg is made apparent in various ways. In amphibians and
chondrostei a grey crescent is formed. In birds, and in some fish, an
eccentric area pellucida appears. For the germ, the formation of the
grey crescent means the same as that of the area pellucida. Either indicates that in normal conditions the plane of bilateral symmetry is fixed
irreversibly. Much wor±v is still necessary to define the precise nature of
the reaction of symmetrization.
Purposely we have not dealt with the problem of symmetrization of
the mammalian egg, which is so special. The early segregation of the
embryonic material from the extra-embryonic material, and the total
lack of experimentation, do not as yet permit valid interpretation of
J. C L A V E R T
thoroughly studied, the determination of the plane of bilateral symmetry proceeds from an active reaction of the egg (a real morphogenetic movement) which is directed by an external factor. The same
directive factors are always found in Vertebrata (spermatozoon, rotation of orientation, rotation of the egg with its membranes, compression, position of the germ in space). As the case may be, one or
another of these factors normally acts, but they are all able to orientate
experimentally the plane of symmetry in Rana fusca. The reaction of
the egg leads to a visible formation which indicates precociously the
position of the plane of symmetry. Experimentally a plane of bilateral
symmetry can be imposed on the egg, but the egg itself fixes it by a
specific reaction.
Two types of eggs can be distinguished from a temporal point of view:
(1) the egg in which symmetrization starts early (with total segmentation, i.e., Amphibia, Chondrostei) where the plane of symmetry is
determined before segmentation starts.
(2) the egg in which symmetrization is retarded (with discoidal segmentation, i.e., birds, selachii, teleostei), where determination occurs in a
germ partially or totally segmented.
It seems evident that egg structure is important in the distinction of
these two types of eggs whose behaviour is so entirely different. The answer
is to be found in the distribution of the yolk and not in amount of yolk.
This study of the determination of the plane of bilateral symmetry
permits us to eliminate the pre-formation of symmetry, as Spemann
thought. This is proved by the fact that in all types of eggs the plane of
symmetry can be determined experimentally according to a preselected meridian. The equipotentiality of the different areas of the egg
distributed around the axis of polarity (animal-vegetal pole) is a netv
notion brought forth by this study.
Each meridian can form a plane of symmetry and become the
cephalo-caudal axis. Lastly, the position of the organizing centre is
fixed when the plane of bilateral symmetry is determined. This reaction
of the egg is made apparent in various ways. In amphibians and
chondrostei a grey crescent is formed. In birds, and in some fish, an
eccentric area pellucida appears. For the germ, the formation of the
grey crescent means the same as that of the area pellucida. Either indicates that in normal conditions the plane of bilateral symmetry is fixed
irreversibly. Much wor±v is still necessary to define the precise nature of
the reaction of symmetrization.
Purposely we have not dealt with the problem of symmetrization of
the mammalian egg, which is so special. The early segregation of the
embryonic material from the extra-embryonic material, and the total
lack of experimentation, do not as yet permit valid interpretation of