Experiments on the Neural Crest of the Lamprey Embryo

It is characteristic of the early development of the vertebrate animals that the embryo should be formed largely as a result of successive displacements of the cell population created during and after cleavage. Of these the first bring the endoderm and the mesoderm into what are effectively their definitive positions by paths that vary somewhat from one vertebrate to another. They are followed by morphogenetic movements which see the medullary system, originally external, rendered internal; and this process is substantially constant throughout the phylum. Both by virtue of its massiveness, and by the ubiquity and variety of the tissues to which it gives rise, this system would merit the status of a fourth 'germ-layer' were it not that it is above all from the study of its neural crest component that the limitations of the germ-layer concept in vertebrate embryology has been most strikingly demonstrated (de Beer, 1947)Nevertheless, as far as our present knowledge goes, the neural crest alone is an embryonic tissue whose fate in different vertebrate classes shows the constancy to be expected of a ' germ-layer'. Thus it certainly gives rise to the spinal ganglia, parts of the dorsal root ganglia in the head, and some of the sheath cells of the peripheral nervous system. In Amphibia it is known to form the bulk of the cartilages of the splanchnocranium and the anterior portion of the trabeculae cranii, and a similar situation probably exists in selachians (Dohrn, 1902), teleosts (Lopashov, 1944) and birds (Goronowitsch, 1892). In Amphibia, birds and teleosts it is an important, if not the sole, source of chromatophores. For more than one reason it is of interest to know something of the fate of the neural crest in Cyclostomes. In the first place it is possible that the crest was not primitively responsible for the diverse tissues to which it gives rise in the living Gnathostomes. It might therefore be expected that the Cyclostomes would show either the primitive or an intermediate condition, the determination of which would serve as an important pointer to the evolution of the crest. The situation is complicated, however, by the uncertainty that exists on the morphological status to be given to the visceral skeleton and so-called trabeculae cranii in lamprey, the Cyclostome most readily available for embryological study. If, as Rathke (1832) and Balfour (1881) thought, the branchial basket of these animals is not comparable with the visceral skeleton of Gnathostomes, and if, as Sewertzoff (1916) and de Beer (1931, 1937)