The Establishment of Certain Reflex Arcs in Foetal Sheep

Abstract

Sheep embryos first respond to mechanical and electrical stimuli applied to them between the 33rd and 35th day after insemination. At this time movements may be elicited by tapping on the unopened amniotic sac or by faradic currents applied to the spinal cord or to the face. The movements appear to be due to the localized contraction of the muscles in the neck, the fore or the hind limbs. The actual neural elements concerned in the production of the movements of the limbs are not definitely known, but it is clear that these movements themselves are not as yet a part of a total mass reaction. 1

If the movements of the limbs in these embryos could be shown to involve at least a primary sensory neuron and a motor neuron, the observations would lend support to the general thesis that behavior has its genesis in individual reflexes which are later associated into reaction patterns. We have, therefore, sought to produce isolated movements of the type observed in the sheep embryos under conditions in which we were certain a primary afferent neuron was stimulated as the first element in a reflex arc.

Mechanical stimuli such as stroking the forelimb with a glass rod or flipping it do not induce either flexion or extension movements in sheep fetuses until 44–47 days after insemination. Reflexes in the hind legs cannot be elicited in these ways until 3 or 4 days later. There remains in these cases the possibility that the stimuli were not adequate in younger embryos to produce reflexes in the limbs.

Fortunately fetuses 42 days old and slightly younger are large enough to be manipulated and dissected. We have found it possible to dissect out and to section the median nerve and also the lateral popliteal.