Notes on the Development of the Neural Crest in the Head of Mus musculus by BENGT KALLEN

T H E relations between the neural crest on the one hand and the neuromeres and their precursors the proneuromeres on the other hand have never been fully described. Many authors have, however, shown that the cranial ganglia lie level with every second neuromere, so that ganglion V lies level with neuromere VII, ganglion VII-VIII level with neuromere IX, and ganglion IX-X level with neuromere XI (Bergquist, 1952). The neuromeres develop in a rostro-caudal wave, which is preceded by a similar wave of proneuromeres—formations of approximately the same appearance but relatively larger (Kallen & Lindskog, 1953). The first proneuromere directly corresponds to neuromeres I—II, the second to neuromeres III-IV, the third to neuromere V; and in the rhombencephalon each proneuromere also corresponds to two neuromeres. These observations raise the following questions: (1) Does the neural crest differentiate in a rostro-caudal sequence as the proneuromeres and neuromeres do? (2) What are the spatial relations between the proneuromeres and the ganglion anlagen of the neural crest? (3) What is the time relation between the formation of the neural crest and the development of the proneuromeres and neuromeres? Adelmann (1925) described the development of the neural crest in the rat as a cranio-caudal progression of proliferation. In a 5-somite embryo the neural crest is only developed in the territory of the mesencephalon and the rhombomere Ai (corresponding to Kallen & Lindskog's proneuromere 4), and forms the anlage of the trigeminus ganglion. In the 8-somite stage there is also an anlage of the facialis-acusticus ganglion and of the glossopharyngicus-vagus ganglion, the latter being continuous with the spinal neural crest. Adelmann considered that there is a cephalo-caudal differentiation also in the spinal neural crest, but he thought that this is independent of that of the head neural crest. He observed a more advanced differentiation in the rostral part of the spinal crest than in the glossopharyngicus-vagus crest.