July i, 1S84] 



NA TURE 



2 ^5 



himself "the right of examining it further," Hansen observes 

 that "it possesses all the reactions of the usual chlorophyll 

 sauce." ' 



In a second paper "• Hansen figures the spectra of chlorophyll 

 green and chlorophyll yellow. His researches will, no doubt, 

 be found useful by students of vegetable chromatology. 



C. A. MacMunn 



RECENT MORPHOLOGICAL SPECULATIONS* 

 \l.—The Origin of Vertibrata 



T7IFTEEN or sixteen years ago Kowalevsky's researches on 

 the development of Amphioxus and of Ascidians seemed 

 to be solving the question of the origin of Vertebrates. The 

 discovery of the larval notochord in Ascidians, and the recog- 

 nition of the homology of their pharyngeal clefts with the gill- 

 slits of Vertebrates, made it necessary to acknowledge the close 

 relationship of the two, as had been already foreshadowed by 

 Herbert Spencer ; while the yet undisputed affinity of Ascidians 

 to Mollusks brought Vertebrates and Invertebrates together in 

 an unbroken line. But as new knowledge brought Ascidians 

 closer to Vertebrates, it undermined their claims to molluscan 

 affinities ; and as the doctrine of degeneration grew up, in 

 the hands of Dohrn and Lankester, it taught that Ascidians, 

 and Amphioxus too for that matter, were not really ancestors of 

 the higher Vertebrata, but only degenerate descendants of such 

 ancestors, poor cousins, as it were, of the higher Vertebrates. 

 The lines by which Vertebrates had sprung from Invertebrates, 

 the common ancestor of Ascidians, Amphioxus, and the higher 

 Vertebrates, had still to be sought for. 



Two leading theories have been formulated, and are still under 

 discussion. The first, identified with the names of Semper and 

 Dohrn, maintains that the nearest allies of the Vertebrates must 

 be looked for among the Chaetopod worms, the dorsal surface 

 and spinal cord of the former corresponding morphologically 

 with the ventral surface of the latter, and its gangliated nerve- 

 cord. On the second view, with which we may associate the 

 names of Balfour and Hubrecht, we must take the ancestor of 

 the Vertebrates to have been some segmented worm, descended 

 from the same unsegment,ed types as the Chaetopods, but in which 

 the two nerve-cords, at first lateral like those of Nemertines, 

 had coalesced dorsally instead of ventrally, to form a median 

 nervous system. 



Our discussion of the first of these theories may be 

 made clearer if we use the words "neural" and "haemal" 

 instead of "dorsal " and "ventral," for the gist of the theory is 

 that in the two groups neural and hccnal surfaces remain con- 

 stant, but what is dorsal in the one is ventral in the other. 



In the Chaetopods, say the advocates of this theory, we have 

 a group of regularly segmented animals, not so far specialised 

 but that we might well conceive ancestors like them to have 

 developed into Vertebrates ; they point to the relations of the 

 nervous, vascular, and alimentary systems, and to the develop- 

 ment of the mesoblast, as being closely parallel in the two 

 groups ; and they try to find traces or representatives in Chaeto- 

 pods of such typically Vertebrate possessions as notochord, gill- 

 clefts, and swimming-bladder. 



At the very outset a difficulty arises which is perhaps the 

 greatest the theory encounters. The mouth of Chaetopods 

 is neurally placed, and surrounded by a nerve-ring ; in Verte- 

 brates it is haemal, and it does not pierce any part of the nervous 

 axis. Dohrn has attempted to overcome this objection. The 

 present mouth of Vertebrates, he says, is not identical with the 

 Invertebrate mouth ; it is a distinct and secondary structure ; it 

 arises late in development, whereas in other classes the " stomo- 

 daeum " or primitive oral invagination appears very early. More- 

 over, in the majority of Vertebrates the mouth does not persist 

 in the position it first appears in ; it arises some way off from the 

 anterior end of the body, and in Elasmobranchs, some Ganoids, 

 and Myxinoids it remains there, but in all other Vertebrates it 

 becomes terminal. If we assume, then, that the mouth in exist- 

 ing Vertebrates is secondary, there must have been a time when 

 it did not exist, and when its functions were performed by another 

 or primary mouth. It has been suggested that in the hypophysis 

 cerebri or " pituitary body" we have, possibly, a remnant of this 

 primary mouth. The hypophysis cerebri appears first as an 

 ectodermic involution, usually arising from the stomodaeum ; but 



1 Tschirsch only obtained his chlorophyll in the form of " blackish-green 

 drops." z Loc. cit. 3 Continued from p. 69. 



in the lamprey, Gotte, Scott, and Dohrn have shown that it 

 arises from the ectoderm which lies anterior to the mouth. It is 

 here, in fact, a little pit of ectoderm, placed between those other 

 two ectodermic pits, which are to become the nose and the 

 mouth. 



If this involution ever pierced the brain and opened upon the 

 neural surface, the fore-brain would then be evidently homolo- 

 gous with the supra-oesophageal ganglion of Invertebrates, or 

 ganglion of the prae-oral lobe. A great deal may be said for tints 

 regarding the fore-brain as distinct from the remaining nervous 

 system ; it resembles the supra-cesophageal ganglion of the In- 

 vertebrata in its close connection with the optic and olfactory 

 organs, and in supplying only organs of sense. There is evidence 

 to show that the third nerve belongs to the cranio-spinal series 

 of segmental nerves, and that the olfactory and optic nerves have 

 a different nature. If this be so, the mid-brain, giving origin 

 to the third nerve, would appear not to have part in the ganglion 

 of the prae-oral lobe. The termination of the notochord directly 

 behind the fore-brain is an additional argument in favour of the 

 morphological distinctness of the latter structure. 



Thus if we follow back the genealogical record of the Verte- 

 brates, we find that at one period their ancestors had a mouth 

 upon the neural surface ; later, two openings into the alimentary 

 canal appear, one on the neural and one on the haemal surface ; 

 still later the latter gains the ascendancy, and alone remains to 

 the present time. This secondary mouth must have arisen from 

 some pre-existing structure ; it could not have originated as a 

 simple depression of the outer skin which deepened and ulti- 

 mately fused with the alimentary canal ; and the only pre-existing 

 organs which could furnish such a passage from the exterior into 

 the alimentary tract are the gill-slits. We must conceive this 

 Vertebrate ancestor as an animal with an intestine which opened 

 anteriorly by a median mouth on its neural surface, and laterally 

 by a series of segmentally situated gill-slits. The mouth took in 

 water, which flowed out over the gill-arches just as it does still 

 in the lower Vertebrates. If from any reason, such as the animal 

 lying like the modern Annelids on its neural surface, it obtained 

 a purer supply of water by taking it in through some of the gill- 

 slits, it is conceivable that a pair of these slits assumed that 

 office, and that by the exercise of this power the gill-slits became 

 gradually larger, and ultimately fused in the middle line. The 

 suctorial power thus acquired to take in water for the purposes 

 of respiration was also of use in obtaining food, and thus a 

 median haemal suctorial mouth arose, such as the Myxinoids now 

 possess. There is much evidence to show that the ancestral 

 Vertebrate possessed a suctorial mouth which subsequently became 

 modified for biting, and was carried forward to the front of the 

 head. Embryology supplies the following arguments in favour 

 of regarding the mouth as formed from the coalescence of a pair 

 of gill-slits. It lies close against the gill-slits, it is separated 

 from them by a gill-arch, it arises about the same time in the 

 embryo, it opens into the alimentary canal ; finally, in some 

 Teleosteans, Belone, Hippocampus, and Gobius, the mouth first 

 appears as two lateral openings, which afterwards fuse in the 

 middle line. 



Admitting that the mouth is formed of two gill-slits, we have 

 to see from what structures in an Annelid such gill-slits could be 

 derived. In many Chaetopods no part of the body is set apart to 

 perform the function of respiration. Where there are no gills 

 the blood is commonly aerated in the walls of the alimentary 

 canal, water being taken in at either end, and when charged 

 with the waste products of respiration, it is expelled through the 

 same opening. In some cases, as in Hesionc, the surface with 

 which the water comes in contact is increased by a pair of lateral 

 sacs or diverticula. It is obvious that with such a respiratory 

 apparatus it would be advantageous if there was an exit for 

 the respired water distinct from' its entrance, so that the blood 

 should always be in contact with pure water. Such an exit 

 would be formed by fusion of the respiratory diverticula with the 

 body-wall anl subsequent rupture of the latter at the points of 

 fusion. And the apertures in the tentacles of Actiniae and the per- 

 forated liver-diverticula of Eolis are adduced as analogous in- 

 stances of such perforation. 



Another suggestion which has been made to account for the 

 origin of gill-slits is that the inner ends of some of the segmental 

 organs gained an entrance into the alimentary tract, and, changing 

 their function, gave rise to gill-slits. 



By these steps a Vertebrate has been reduced to an Annelid 

 structure, but certain questions which have arisen in the develop- 

 ment of this theory remain to be answered. One is whether the 



