Sept. 2, 1880] 



NATURE 



419 



layer, known as the mesoblast, early makes its appearance 

 between the two primaiy layers. The mesoblast originates as a 

 differentiation of one or of both the primary germinal layers ; 

 but although the difl'erent views which have been held as to its 

 mode of origin form an important section of the history of 

 recent embryological investigations, I must for the moment 

 confine myself to saying that from this layer there take their 

 origin — the whole of the muscular system, of the vascular 

 system, and of that connective-tissue system which forms the 

 internal skeleton, tendons, and other parts. 



We have seen that the epiblast represents the skin or epi- 

 dermis of the simple sac-like ancestor common to all the 

 Metazoa. In all the higher Metazoa it gives rise, as might be 

 expected, to the epidermis, but it gives rise at the same time to 

 a number of other organs ; and, in accordance with the principles 

 laid down in the earlier part of my address, it is to be concluded 

 that the organs so derived have hcen formed as di/ferentiaiwns of the 

 primitive epidermis. One of the most interesting of recent 

 embryological discoveries is the fact that the nervous system i?, 

 in all but a very few doubtful cases, derived from the epiblast. 

 This fact was made out for vertebrate animals by the great 

 embryologist Van Baer ; and the Russian naturalist Kowalevsky, 

 to whose researches I have already alluded, showed that this 

 was true for a large number of invertebrate animals. The 

 derivation of the nervous jystem from the epiblast has since 

 been made out for a sufficient number of forms satisfactorily to 

 establish the generalisation that it is all but universally derived 

 from the epiblast. 



In any animal in which there is no distinct nervous system it 

 is obvious that the general surface of the body must be sensitive 

 to the action of its surroundings, or to what are technically 

 called stimuli. We know experimentally that this is -so in the 

 case of the Protozoa, and of some vei"y simple Metazoa, such as 

 the fresh-water Polype or Hydra, where there is no distinct 

 nervous system. The skin or epidermis of the ancestor of the 

 Metazoa was no doubt similarly sensitive ; and the fact of the 

 nervous system being derived from the epiblast implies that the 

 functions of the central nervous system, which were originally 

 taken by the whole skin, became gradually concentrated in a 

 special part of the skin which was step by step removed from 

 the surface, and finally became a well-defined organ in the 

 interior of the body. 



What were the steps by w hich this remarkable process took 

 place ? How has it come about that there are nerves passing 

 from the central nervous system to all parts of the skin, and 

 also to the muscles? How have the arrangements for reflex 

 actions arisen by which stimuli received on the surface of the 

 body are carried to the central part of the nervous system, and 

 are thence transmitted to the appropriate muscles, and cause 

 them to contract ? All these que-tions require to be answered 

 before we can be said to possess a satisfactory knowledge of the 

 origin of the nervous system. As yet, however, the knowledge 

 of these points derived from embryology is imperfect, although 

 there is every hope that further investigation will render it less 

 so? Fortunately, however, a study of comparative anatomy, 

 especially that of the Cctlenterata, fills up some of the gaps left 

 from our study of embryology. 



From embryology we learn that the ganglion-cells of the 

 central part of the nervous system are originally derived fronr 

 the simple undifferentiated epithelial cells of the surface of the 

 body. We further learn that the nerves are out-growths of the 

 central nervous system. It was supposed till quite recently that 

 tlie nerves in Vertebrates were derived from parts of the middle 

 germinal layer or mesoblast, and that they only became secon- 

 darily connecte 1 with the central nervous system. This is now- 

 known not to be the case, but the nerves are formed as processes 

 growing out from the central part of the nervous system. 



Another important fact shown by embryology is that the 

 central nervous system, and percipient portion of the organs of 

 special sense, are often formed from the same part of the 

 primiiive epidermis. Thus, in ourselves and in other vertebrate 

 animals the sensitive part of the eye, known as the retina, is 

 formed from two In.teral lobes of the front part of the primitive 

 brain. The cr)-stalline lens and cornea of the eye are, however, 

 subsequently formed from the skin. 



The same is true for the peculiar compound eyes of crabs or 

 Crustacea. The most important part of the central nervous 

 system of these animals is the suprao;sophageal ganglia, often 

 known as the brain, and these are formed in the embryo from 

 two thickened patches of the skin at the front end of the body. 



These thickened patches become gradually detached from the 

 surface, remaining covered over by a layer of skin. They then 

 constitute the supraffisophageal ganglia ; but they form not only 

 the ganglia, but also the rhabdons or retinal elements of the eye 

 — the parts in fact which correspond to the rods and cones in 

 our owm retina. The layer of epidermis or skin which lies 

 immediately above the suprao;sophageal ganglia becomes gra- 

 dually converted into the refractive media of the crustacean eye. 

 A cuticle which lies on its surface forms the peculiar facets on 

 the surface of the eye, which are known as the corneal lenses, 

 while the cells of the epidermis give rise to lens-like bodies 

 known as the crystalline cones. 



It would be easy to quote further instances of the same kind, 

 but I Irast that the two which I have given will be sufficient to 

 show the kind of relation which often exists between the organs 

 of special sense, especially those of vision, and the central 

 nervous system. It might have been anticipated a priori that 

 organs of special sense would only appear in animals provided 

 with a well-developed central nervous system. This, however, 

 is not the case. Special cells with long delicate hairs, which 

 are undoubtedly highly sensitive structures, are present in 

 animals in which as yet nothing has been found which could be 

 called a central nervous system ; and there is every reason to 

 think that the organs of special sense ox\%vaX&ii pari passu with 

 the central nervous system. It is probable that in the simplest 

 organisms tlie whole body is sensitive to light, but that with the 

 appearance of pigment-cells in certain parts of the body, the 

 sensitiveness to light became localised to the areas where the 

 pigment-cells were present. Since, however, it was necessary 

 that stimuli received by such organs should be communicated 

 to other parts of the bod)', some of the epidermic cells 

 in the neighbourhood of the pigment-spots, which were at 

 first only sensitive, in the same manner as other cells of the 

 epidermis, became gradually differentiated into special nerve- 

 cells. As to the details of this differentiation, embryology 

 does not as yet throw any great light ; but from the study 

 of comparative anatomy there are grounds for thinking that 

 it was somewhat as follows : — Cells placed on the surface sent 

 protoplasmic processes of a nervous nature inwards, which 

 came into connection with nervous processes from similar cells 

 placed in other parts of the body. The cells with such pro- 

 cesses then became removed from the surface, forming a deep 

 layer of the epidermis below the sensitive cells of the organ of 

 vision. With these cells they remained connected by proto- 

 plasmic filaments, and thus they came to form a thickening of 

 the epidermis underneath the organ of vision, the cells of which 

 received their stimuli from those of the organ of vision, and 

 transmitted the stimuli so received to other parts of the body. Such 

 a thickening would obviously be the rudiment of a central nervous 

 system, and it is easy to see by what steps it might become 

 Gradually larger and more important, and might gradually travel 

 inwards, remaining connected with the sense-organ at tlie surface 

 by protoplasmic filaments, which would then constitute nerves. 

 The rudimentary eye would at first merely consist partly of cells 

 sensitive to light, and partly of optical structures constituting the 

 lens, which would throw an image of external objects upon it, 

 and so convert the whole structure into a true organ of vision. 

 It has tims come about that, in the development of the indi- 

 vidual, the retina or sensitive part of the eye is first formed in 

 connection with the central nervous system, while the lenses of 

 the eye are independently evolved from the epidermis at a later 

 period. 



The general features of the origin of the nervous system 

 which liave so far been made out by means of the study of 

 embryology are the following : — 



1. That the nervous system of the higher Metazoa has been 

 developed in the course of a long series of generations by a 

 tT.adual process of differentiation of parts of the epidermis. 



2. That part of the central nervous system of many forms 

 arose as a local collection of nerve-cells in the epidermis, in the 

 neighbourhood of rudimentary organs of vision. 



3! That ganglion-cells have been evolved from simple epi- 

 thelial cells of the epidermis. 



4. That the primitive nerves were outgrowths of the original 

 ganglion cells; and that the nerves of the higher forms are 

 formed as outgrowths of the central nervous system. 



The points on which embryology has not yet thrown a 

 satisfactory light are : — 



I. The steps by which the protoplasmic processes, from the 

 primitive epidermic cells, became united together so as to form 



