On the Nature of the First Forelimb Movements of Mammalian Embryos

Somatic movements occurring in response to faradic stimulation were studied in cat embryos delivered by experimental Caesarian section. The cats were decerebrated by ligating carotid and basilar arteries, after which one to 3 hours were allowed for recovery from ether anesthesia. Embryos were delivered as promptly as possible and examined while their placental circulation was intact and before uterine contractions had caused placental separation. All experimentation was carried out in a bath of physiologic saline solution maintained at the temperature of the cat's body. The combined diameter of the nichrome-wire stimulating electrodes was less than one millimeter and they were insulated except at their tips. Strength of current was not measured but could be borne upon the tongue. At the conclusion of physiologic studies, the embryos were prepared for histologic examination by the pyridine-silver technique. It was seldom possible to obtain reflex movements in embryos less than 14 mm. long. But when the electrodes were placed directly over developing muscle masses of the shoulder region contractions were elicited with perfect regularity. Motor and sensory spinal nerves are well formed at this stage; free nerve endings appear beneath the epithelium of the forelimb and a few primitive endings are present upon the myoblasts of the shoulder muscles. Within the spinal cord, motor and associational mechanisms are well represented but morphologic reflex arcs are incomplete because the sensory dorsal funiculus collaterals are just forming and have made no connections with other neurons. In embryos of about 23 days' insemination age (14 mm.), the sensory nerves have grown down the dorsal and volar aspects of the forelimb to within half a millimeter of its most distal point. There they end in free nerve endings immediately beneath the epithelium.