200
J. MILAIRE
of the marginal venous sinus, which lies between it and the ectodermal
ridge. Consequently, this vessel becomes narrower opposite each metapodial precartilage while it remains wide in the intermediate interdigital
spaces. The marginal ectoderm itself soon becomes raised at the site of
each growing precartilage; this changes the semi-circular outline of the
footplate to a polygonal one (Fig. 5(a, b)). At the same time, the a.e.r.
acquires the normal thickness of the rest of the ectoderm, though it
retains for some time its histochemical peculiarities. In the mole embryo,
the only species in which these stages have been studied in detail, the
alkaline phosphatase disappears in the interdigital zones of the a.e.r.
but persists at the head of each growing digital bud. It thus seems that
specific activity is maintained in these restricted ectodermal areas,
probably as a result of the persistence of some localized mesodermal
'maintenance factor' in the corresponding precartilages. The growing
digital buds contain RNA and, in the mole embryo, glycogen (Fig. 6(b)).
Some other histochemical features observed in the mesoderm of the
mouse and the mole limb buds are suggestive of a vanishing ectodermal
influence in the interdigital spaces. As soon as the digital buds become
visible, the outer mesodermal cells underlying the interdigital zones of
the ectoderm undergo a process of degeneration. The very early stages
of this process can be shown by the Unna-Brachet technique which
demonstrates the formation of droplets of RNA and DNA in the cytoplasm of the degenerating cells, and, at a later stage, in their nuclei.
The cytoplasmic droplets simultaneously acquire acid phosphatase
which was absent in the healthy cells. These cells changes first occur in
only two or three layers underlying each interdigital zone of the ectoderm. Later, the degenerative process spreads proximally, involving
more and more cells in the interdigital area (Fig. 4(a, b)). The existence of
degenerative phenomena in the interdigital mesoderm was previously
reported in the limb buds of the chick (Saunders et al., 1957b) and the
mouse (Forsthoefel, 1959) embryos, but no details were given. The
topographical spreading of the histochemical changes occurring in the
degenerating cells from distal to more proximal zones of the mesoderm
strongly suggests that the more proximal the cells, the longer they are
subject to the ectodermal influence.
b. Chronology of chondrification
After the first two proximal precartilages have formed in the proximal
part of the limb bud and have started to synthesize an extracellular
substance containing mucopolysaccharides, both zeugopodial precartilages condense simultaneously at a more distal level and show the first
histochemical signs of chondrification. Although the axial artery of the
limb bud is running in a proximo-distal direction through the meso-
J. MILAIRE
of the marginal venous sinus, which lies between it and the ectodermal
ridge. Consequently, this vessel becomes narrower opposite each metapodial precartilage while it remains wide in the intermediate interdigital
spaces. The marginal ectoderm itself soon becomes raised at the site of
each growing precartilage; this changes the semi-circular outline of the
footplate to a polygonal one (Fig. 5(a, b)). At the same time, the a.e.r.
acquires the normal thickness of the rest of the ectoderm, though it
retains for some time its histochemical peculiarities. In the mole embryo,
the only species in which these stages have been studied in detail, the
alkaline phosphatase disappears in the interdigital zones of the a.e.r.
but persists at the head of each growing digital bud. It thus seems that
specific activity is maintained in these restricted ectodermal areas,
probably as a result of the persistence of some localized mesodermal
'maintenance factor' in the corresponding precartilages. The growing
digital buds contain RNA and, in the mole embryo, glycogen (Fig. 6(b)).
Some other histochemical features observed in the mesoderm of the
mouse and the mole limb buds are suggestive of a vanishing ectodermal
influence in the interdigital spaces. As soon as the digital buds become
visible, the outer mesodermal cells underlying the interdigital zones of
the ectoderm undergo a process of degeneration. The very early stages
of this process can be shown by the Unna-Brachet technique which
demonstrates the formation of droplets of RNA and DNA in the cytoplasm of the degenerating cells, and, at a later stage, in their nuclei.
The cytoplasmic droplets simultaneously acquire acid phosphatase
which was absent in the healthy cells. These cells changes first occur in
only two or three layers underlying each interdigital zone of the ectoderm. Later, the degenerative process spreads proximally, involving
more and more cells in the interdigital area (Fig. 4(a, b)). The existence of
degenerative phenomena in the interdigital mesoderm was previously
reported in the limb buds of the chick (Saunders et al., 1957b) and the
mouse (Forsthoefel, 1959) embryos, but no details were given. The
topographical spreading of the histochemical changes occurring in the
degenerating cells from distal to more proximal zones of the mesoderm
strongly suggests that the more proximal the cells, the longer they are
subject to the ectodermal influence.
b. Chronology of chondrification
After the first two proximal precartilages have formed in the proximal
part of the limb bud and have started to synthesize an extracellular
substance containing mucopolysaccharides, both zeugopodial precartilages condense simultaneously at a more distal level and show the first
histochemical signs of chondrification. Although the axial artery of the
limb bud is running in a proximo-distal direction through the meso-