[Development of the dorsal branches of the spinal nerves during the true embryonic period].

A study of the Carnegie collection of human embryos has enabled the author to establish the sequence of events in the development of the dorsal rami of the spinal nerves. As the ventral rami enter the muscle anlagen and the rami communicantes appear at stage 14, it is sometimes possible to note the anlage of a dorsal ramus in the cervical region. At stage 15, the dorsal ramus is well distinguished, but seems to correspond with only its lateral division; indeed, the medial division is evident only in the next stage. At stage 17, the lateral and medial branches exist throughout the entire length of the body. It is only at stage 19, that nerves fibres reach beyond the myotome to the fascia. The next stage does not present any specific feature. At stage 21, however, fibres of the lateral ramus pass beyond the b=fascia and reach the cutaneous vascular layer, whereas fibres of the medial ramus have not yet gained the fascia. At the end of the embryonic period proper (stage 23), the cutaneous fibres of the lateral branch begin to cling to the corium (dermis), but the medial fibres seem merely to pass through the vascular layer without any attachment to the chorion. In summary, it can be stated that: 1) the dorsal rami of the spinal nerves are a little late in comparison with the lateral ones, 2) their lateral branches precede the medial ones, 3) they develop steadily from the depth to the surface, from the cervical to the caudal region, and from the lateral to the medial area of the dorsal skin.