226 



NA TURE 



[July 3, 1884 



month is formed from the most anterior pair of gill-slits. If the 

 trabecular cranii are gill-arches, the mouth is not the first. Some 

 authorities consider the nasal sacs as modified gill-slits ; they are 

 primitively double, and where we find them single, as in Amphi- 

 oxus and Cyclostomes, it is due to secondary processes. 



In his " Monograph on the Development of the Elasmobranch 

 Fishes," Balfour lias pointed out that the histological structure 

 of the spinal cord in Vertebrates is exactly what would be found 

 if, by mechanical folding, the two lateral halves of the nerve-cord 

 of an Annelid became bent toward one another, whilst the external 

 skin was pushed into the groove between them. If this folding 

 were completed, so that the external epithelium formed a canal 

 surrounded by nervous tissues, the white and gray matter would 

 assume the same relative position that they possess in the spinal 

 cord of Vertebrates. The nerves would then arise not laterally, 

 but from the extreme ventral summit, and would thus correspond 

 with the posterior roots of the Vertebrate spinal cord, which, as 

 Balfour has shown, grow out from the extreme dorsal summit ol 

 the neural canal, a position comparable with the ventral summit 

 of the Annelid nervous system. In Amphibia the primitive 

 medullary plate (or modified area of dorsal epiblast which is to 

 fold in and form the medullary groove), although elsewhere 

 single, shows signs of being formed of two symmetrical halves. 

 and in both embryo and adult the neural tube has a structure 

 which points to its origin from the coalescence of two lateral 

 cords. 



The direction of the blood current, which flows from behind 

 forwards on thehaemal, and from before backwards on the neural, 

 surface, agrees in Chaetopods and Vertebrates if the surfaces be 

 reversed, and the hypothesis of reversal presents no great diffi- 

 culties in the case of a cylindrical animal swimming in the sea. 



In connection with this theory it is interesting to note that 

 Eisig has instituted a comparison between the lateral sense-organs 

 in the Capitellidse (a family of Chsetopods) and the lateral line of 

 fishes, anil he further compares the "siphon" of the same 

 Chsetopods with that obscure roil of tissue split off from the 

 alimentary tract of fishes and Amphibia, the sub-notochordal rod. 



The notochord is one of the most characteristic Vertebrate 

 structures, and if the theory propounded above be true, we should 

 expect to find very distinct rudiments of such a structure amongst 

 the Chastopods, but although numerous organs have been inter- 

 preted as such, Balfour states that none of these interpretations 

 will bear examination. Quite recently Xussbaum has found in 

 the cockroach a longitudinal string of cells lying upon the nerve- 

 chain, which in its development bears a striking resemblance to 

 the notochord of Vertebrates, and Vejdovsky has described a 

 similar structure in Oligochaeta, developed, however, from the 

 mesoderm, under the name of neurochord. 



The supporters ol the second theory, which we have connected 

 with the names of Balfour and Ilubrecht, claim that they have 

 found an organ in one class of the Invertebrata which is com- 

 parable to the notochord of Vertebrata. 



Balfour in the "Elasmobranch fishes." whilst combating the 

 Chaetopod origin of Vertebrates, suggested that Vertebrates have 

 descended from the same unsegmented stock as the Chsetopods, 

 but through some other line which has entirely disappeared. 

 They have thus acquired similar segmental and other organs. 

 In this line of ancestors he imagines that the primitive lateral 

 nerve cords have tended to coalesce dorsally instead of ventrally. 

 In his "Comparative Embryology" he repeats these views, and 

 adds that their probability has been increased by the researches 

 of Hubrecht, who has shown that in some Nemertines the nerve- 

 cords approach each other very closely in the median dorsal 

 line. Ilubrecht has epiite recently amplified these views by 

 suggesting that "the proboscis of Nemertines, which arises as 

 an invaginable structure, and which passes through a part of the 

 cerebral ganglion, is homologous with that rudimentary organ 

 which is found in the whole series of Vertebrates without excep 

 tion — the hypophysis cerebri. The proboscidian sheath of the 

 Nemertines is comparable in situation (and development?) with 

 the notochord of Vertebrates." 



The first of these two positions is supported by the facts of 

 development. Although the details of the ontogenetic origin of 

 the Ncmertine proboscis are still wanting, the broad fact that it 

 arises, like the hypophysis cerebri, as an invagination of the epi- 

 blast, has been established. 



These organs further resemble each other in the shifting of 

 their external opening, which is in sonic cases "ii the outer sur- 

 face, in others on the dorsal \sall of the alimentary canal just 

 within the mouth. 



In this comparison between the proboscis of Nemertines and the 

 hypophysis cerebri, the connection of the latter with the brain and 

 its relation to the anterior end of the notochord, must be especially 

 borne in mind. The proboscis passesbackward, between the anterior 

 thickenings which form the brain, the two lateral halves of which 

 are connected by a thick nerve commissure ventral to the proboscis, 

 and by a thin strand dorsal to it. Thus the proboscis pierces a 

 ring of nervous tissue, and the proboscis sheath reaches forward 

 to the level of this nervous commissure. This region, then, 

 would correspond to that part of the vertebrate brain to which 

 the hypophysis cerebri is attached and close behind which the 

 notochord terminates, and would thus separate off the fore-brain 

 from the remaining nervous system. In connection with thi^ it 

 is a very significant fact that the superior lobes of the Nemertine 

 brain give rise to the nerves which supply the sense organs, 

 while the strong nerve which supplies the anterior region of the 

 oesophagus originates in the inferior lobes. The one pair of 

 these lobes may thus have been perpetuated as the fore-brain, 

 and the other as the rest of the nervous system. 



The sheath of the proboscis corresponds very accurately in it^ 

 position to the notochord, but unfortunately the knowledge we 

 possess of its development is not great. Barrois and Salensky 

 have attributed a mesoblastic origin to it, the latter, however, 

 noting a connection between the first origin of the oesophagus 

 and proboscis. Hoffman has stated that part of the proboscis 

 is split off the dorsal surface of the alimentary canal, whilst the 

 muscular proboscis sheath is mesoblastic in origin. Hubrecht 

 suggests that possibly the formation of the inner part of the 

 proboscis sheath has been mistaken for the proboscis. If this 

 suggestion prove true, then the proboscis sheath agrees both in 

 position and origin with the notochord of Vertebrates. 



The fully-developed notochord is a solid rod, whereas the 

 proboscis sheath is a hollow tube. This, however, is no very 

 serious objection to their homology; and recently Lieberkuhn 

 and Braun have shown that the notochord arises at first as a 

 hollow tubiform structure, whilst in old specimens of Cerebra- 

 lulns (a Nemertine) the posterior end of the proboscis sheath is 

 nearly or quite filled up with continuous cellular tissue. 



We have unfortunately too little knowledge at present to in- 

 stitute a comparison between the other organs of Nemertines 

 and Vertebrates. Attention should, however, be called to the 

 ciliated lateral pits upon the head. These arise from the most 

 anterior part of the oesophagus in front of the mouth. They 

 bud out from the walls of the oesophagus, and are in this stage 

 directly comparable to similar diverticula which arise in the same 

 region in the larva of Balauoglossus, and which there give rise- 

 to the first pair of branchial slits. These diverticula become 

 finally cut off from the oesophagus, but enter into connection 

 with epiblastic invaginations, and are thus placed in communica- 

 tion with the sea-water. In Schizonemertines their inner end is 

 in connection with the brain; the latter contains haemoglobin, 

 and so they subserve respiration. In Hoplonemertines, how- 

 ever, although their development is similar, they apparently arc- 

 modified for a sensory, possibly an olfactory, function. In con- 

 nection with these structures, Hubrecht calls attention to some 

 of the results of Hatschek's recent researches on the develop- 

 ment of Amphioxus. In this animal there are two lateral hypo- 

 blasts diverticula growing out from the anterior part of the 

 oesophagus in front of the mouth. These differ both in their 

 nature and development from the archenteric diverticula, or from 

 the branchial outgrowths. They are at first symmetrical. 1ml 

 have a different fate. They are both constricted off from the 

 hypoblast: the left one communicates with the exterior by a 

 ciliated opening which appears in the epiblast ; the right one 

 forms an epithelial lining to the prseoral body region. The left 

 one has been looked upon as a special sen^c organ of the larva. 



Finally it is impossible to overlook the bearing of l'.alano- 

 gli issus on our subject, although we are not yet in possession of 

 all the facts that Mr. Bateson's (QJ.M.S. No. xciv. April 18S4) 

 recent researches seem to have elicited. The pharyngeal slits of 

 balauoglossus have long been recognised as wonderfully like the 

 gill-slits of Vertebrates, and on the other hand as totally unlike 

 any structures possessed by animals outside the Chordata. But 

 Bateson's researches have already shown that the developmental 

 features of the nervous system and of the mesoblast are not less 

 suggestive of the same kinship. For the mesoblast is developed 

 from an anterior archenteric pouch with two posterior horns 

 (exactly comparable with that described in the last paragraph as 

 existing in Amphioxus) and two pairs of posterior pouches instead 

 of the larger number that Amphioxus possesses. And the fate 



