L I M B M O R P H O G E N E S I S I N V E R T E B R A T E S
187
of the mesoderm continues at each end of the crest where it gives rise
to the limb buds themselves, while the intermediate part of the crest is
progressively included into the lateral wall of the embryo. The Wolff
crest stage may be regarded as the earliest stage of limb development.
1. Wolff Crest Stage
The flbroblast-like mesodermal cells proceeding from the somatopleure leave their original layer both by migration and by mitotic
activity. In all species, they show a high RNA content in the whole
length of Wolff's crest and do not show any histochemical peculiarity at
the levels where the limb buds will develop. In the mouse embryo and,
to a greater extent in the mole embryo, the mesoderm of Wolff's crest
also contains alkaline phosphatase. This is present in the somatopleure
of the species at earlier stages.
Although the limb bud mesoderm mainly originates from the dorsal part of the somatopleure, two other secondary origins have been
observed in some forms. In reptiles, the ventral part of each dermatomyotome penetrates into the proximal zones of the limb buds and contributes to their mesoderm (Corning, 1899; Milaire, 1957). In the mole
embryo, strands of neural crest cells containing alkaline phosphatase
seem to penetrate into the mesoderm of the fore-limb bud after they
have invaded the narrow intersomitic spaces.
The structure and the histochemical characteristics of the twolayered ectoderm of Wolff's crest do not differ from those of any other
ectodermal region of the embryo.
2. First Changes in the Early Limb Bud (Induction of the Ventral
Ectoderm)
Two consecutive events occur in the distal part of the limb buds as
soon as they become recognizable at each end of Wolff's crest (the forelimb bud always appearing before the hind-limb bud). The first change
takes place in the outer part of the mesoderm which becomes condensed
and shows a perceptible increase of RNA. In all forms studied, but mostly
in mammals, this process appears to be more important in the ventral
part of the mesoderm than in the dorsal. In some reptiles, the same mesodermal area may also contain glycogen (Milaire, 1957, Figs. 22 and 30,
Plate XVII). Shortly afterwards, a thickening appears in the whole
ventral part of the limb bud ectoderm underlying the condensed ventral
mesoderm, both cuboidal and peridermal cells contributing to this
morphological change of the ectoderm. While the dorsal ectodermal
covering of the limb bud remains thin and does not show any histo-
187
of the mesoderm continues at each end of the crest where it gives rise
to the limb buds themselves, while the intermediate part of the crest is
progressively included into the lateral wall of the embryo. The Wolff
crest stage may be regarded as the earliest stage of limb development.
1. Wolff Crest Stage
The flbroblast-like mesodermal cells proceeding from the somatopleure leave their original layer both by migration and by mitotic
activity. In all species, they show a high RNA content in the whole
length of Wolff's crest and do not show any histochemical peculiarity at
the levels where the limb buds will develop. In the mouse embryo and,
to a greater extent in the mole embryo, the mesoderm of Wolff's crest
also contains alkaline phosphatase. This is present in the somatopleure
of the species at earlier stages.
Although the limb bud mesoderm mainly originates from the dorsal part of the somatopleure, two other secondary origins have been
observed in some forms. In reptiles, the ventral part of each dermatomyotome penetrates into the proximal zones of the limb buds and contributes to their mesoderm (Corning, 1899; Milaire, 1957). In the mole
embryo, strands of neural crest cells containing alkaline phosphatase
seem to penetrate into the mesoderm of the fore-limb bud after they
have invaded the narrow intersomitic spaces.
The structure and the histochemical characteristics of the twolayered ectoderm of Wolff's crest do not differ from those of any other
ectodermal region of the embryo.
2. First Changes in the Early Limb Bud (Induction of the Ventral
Ectoderm)
Two consecutive events occur in the distal part of the limb buds as
soon as they become recognizable at each end of Wolff's crest (the forelimb bud always appearing before the hind-limb bud). The first change
takes place in the outer part of the mesoderm which becomes condensed
and shows a perceptible increase of RNA. In all forms studied, but mostly
in mammals, this process appears to be more important in the ventral
part of the mesoderm than in the dorsal. In some reptiles, the same mesodermal area may also contain glycogen (Milaire, 1957, Figs. 22 and 30,
Plate XVII). Shortly afterwards, a thickening appears in the whole
ventral part of the limb bud ectoderm underlying the condensed ventral
mesoderm, both cuboidal and peridermal cells contributing to this
morphological change of the ectoderm. While the dorsal ectodermal
covering of the limb bud remains thin and does not show any histo-