52 EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS. 



the antecedent sense buds, for the latter are best seen in the embryos of scorpions, 

 while my most detailed work on the cord has been done on Limulus. But the 

 conditions in the two animals are so similar that there can be no reasonable doubt 

 that an arthropod neuromere consists of distinct clusters of nerve cells, each sur- 

 rounded by a special sheath of neuroglia, each projecting its fibers along the same 

 paths to the same terminals, and each directly descended from one or more 

 embryonic sense buds. 



In the early stages of the neuron in vertebrates, as in the late stages in the 

 scorpion, the nerve cells are often arranged in parallel vertical rows, which may 

 be interpreted in the same manner as in arthropods, that is, as the ontogenetic 

 remnants of ancestral sense buds. 



There are many familiar instances where nerve cells arise from the same 

 points in the ectoderm as the sensory ones. It is highly probable, in such cases, 

 that the nerve cells are ultimate phases in the specialization of sense cells. For 

 example, in Acilius, a few cells of large size leave the embryonic retina at a com- 

 paratively late stage; they finally join the optic ganglion and become giant nerve 

 cells, having such a peculiar form and location that they may be readily recognized 

 through life (Patten) . Ganglion cells may also arise from the gustatory epithelium 

 in Limulus, or from the epithelium of lateral line organs in vertebrates. But in 

 none of these cases has it been clearly shown, to my knowledge, that a functional 

 and structurally complete sensory cell is bodily metamorphosed into a ganglion 

 cell. However, just such a metamorphosis as this does take place in Limulus and 

 Branchipus, where the large rod-bearing visual cells are converted into true gang- 

 lion cells, which still retain indications of their primitive grouping into ommatidia 

 and remnants of the visual rods. (p. 162 and Fig. 109, ,4.) 



The transformation of well developed sense buds into ganglion cells, as just 

 described for the neuron of arachnids, is not, therefore, without precedent. 



In most arthropods, the primitive sense buds, while undoubtedly present in 

 some form, are not as well developed as they are in scorpions. Hence certain 

 authors have failed to recognize their real character, and have interpreted them 

 as neuroblasts, or even as nutritive folds, or as folds produced by growth pressure. 

 Such interpretations are untenable. It is true that the sense buds may be repre- 

 sented by small conical groups of cells, or nuclei, arising from the proliferation of 

 a single deep-lying cell, or nucleus, or "neuroblast." But the formation of these 

 neuroblasts is to be regarded as an abbreviated method of repeating the sense bud 

 stage so clearly seen in the scorpion. 



Even these neuroblasts may be omitted, or their appearance postponed to a 

 relatively late embryonic period; the entire cord then has its origin in a few 

 terminal neuroblasts (or telo-neuroblasts), as in Cymothoa. 



