9S 



HA RD WICKE'S SCIENCE- G OS SI P. 



THE STRUCTURE OF INSECTS IN RE- 

 LATION TO THE ORIGIN OF VERTE- 

 BRATES. 



PROFESSOR B. T. LOWNE, F.R.C.S., 

 recently delivered at the Royal College of 

 Surgeons three lectures on thel Structure and 

 Development of the Skeleton of the Head, the 

 Nervous System, and Sensory Organs of Insects in 

 relation to Recent Views on the Origin of Vertebrates, 

 giving the results of work in which he has been 

 engaged for at least fifteen or twenty years. 'His 

 views are that the vertebrate and the arthropod 

 stand in a genetic connection with each other. He 

 said : "If we seek for links uniting these two great 

 sub-kingdoms, we must look for them amongst the 

 most generalised groups in each : in the vertebrata 

 amongst the amphibia, and especially the perenni- 

 branchiate forms ; in the arthropods, and in the king- 

 crab "Limulus," which hold a zoological position 

 between the arachnids and the Crustacea." The 

 lecturer compared the embryos of the axolotl, as 

 figured by Professor Parker with arthropod embryos, 

 and showed many points of similarity. He then 

 /adverted to the fact that both Drs. Gaskell and 

 Patten had independently arrived at the conclu- 

 sion that the king crab stands in a genetic relation 

 to the vertebrata. 



The existence of a notocliord in the chordata has 

 been looked upon as of jDrimary import in the 

 question of the descent of vertebrates from an in- 

 vertebrate type. Adopting, however, Gaskell's 

 views, and holding that the central canal of the 

 .spinal cord represents the arthropod alimentary canal, 

 the lecturer showed at some length that the procto- 

 deum of the insect embryo corresponds with the 

 mesenteron of the vertebrata ; and he held that a 

 notochord should rather be regarded as a secondary 

 character, resulting from an invagination of the 

 epiblast, than as a structure of a high morphological 

 significance ; and he showed that a rod of cartilage, 

 similar to a notochord, is actually developed on the 

 dorsal surface of the invaginated head capsule of the 

 b.lowfly larva, whilst those structures in invertebrates 

 which have been supposed to represent a notochord 

 ■ have a similar origin. He pointed out that the 

 structure which supported the nerve centres in 

 Limulus, is composed of cartilage very like the 

 cartilage of the vertebrata, and it could only, with 

 great difficulty, be distinguished from it. Professor 

 Ray iLankester was the first who made out this 

 peculiar form of cartilage, which he said was developed 

 from the mesoblast ; while the lecturer, judging from 

 the relation the structure bears to the same parts in 

 insects, believed it to be an epiblastic structure. 

 This led him to accept the view put forward by Dr. 

 Patten in America and by Dr. Gaskell in this country, 

 that the ventral surface of the anterior somites of the 

 arthropods represents the basis cranii of the verte- 



brata. The relationship between the nervous 

 system in the vertebrates and the arthropods led him 

 to adopt Dr. Gaskell's hypothesis, inasmuch as it 

 harmonised many things which had formerly 

 puzzled biologists. For many years it had never 

 occurred to him to compare any parts of the arthro- 

 pods with the functional corresponding parts in the 

 vertebrata. As soon as Dr. Gaskell showed that 

 there 'were strong reasons for believing that the 

 central canal of the spinal cord corresponds with the 

 primary intestinal canal, the difficulty considerably 

 diminished, and many parts of the insect were found 

 to correspond so closely to parts of the vertebrate, 

 that it was no longer possible to ignore the corres- 

 pondences between the structures, both in develop- 

 ment and in relation to their anatomical parts. 

 Having given a description of the insect brain, 

 Professor Lowne referred to the idea that the 

 antennae of insects were homologues of the ventral 

 appendages. The evidence against that idea was 

 becoming stronger and stronger every day. They 

 were not lateral appendages, in the usual sense — a 

 fact long since recognised by Professor Balfour, — but 

 olfactory organs, corresponding point by point, more 

 especially in relation to the nerve ganglia, to the 

 olfactory organs of the vertebrata. Neither did the 

 eyes of the insect represent ventral appendages ; they 

 bore no relation whatever to them. 



The lecturer entered at great length into the 

 details of the structure and development of the 

 arthropod brain, and showed that it possesses many 

 points of similarity with the cephalic nerve centres of 

 vertebrates ; that it is developed from three vesicles ; 

 and that median pineal eyes are developed in the 

 wall of the middle vesicle. That there is actually a 

 third ventricle from which the nerve of the median 

 eyes, ocelli, arises. 



The lecturer finally dealt with the development of 

 the eye. The sensory organs of arthropods were 

 usually developed by the process of invagination of 

 the epiblast, just in the same way as the sense 

 capsules of vertebrates were developed. The eyes 

 first appeared as a single layer of cells in a kind of 

 cup-shaped cavity, divided into at least two layers ; 

 from the surface layer a series of lenses was developed 

 about 4000 to 6000 in number, instead of one great 

 lens. In some insects there were as many as 24,000 

 to 30,000 separate lenses. These form the compound 

 cornea. The lenses were very remarkable in their 

 structure, and appeared to consist of a stroma very 

 much like the stroma of the red blood-corpuscle of a 

 vertebrate, and a substance which has a very highly 

 refractive power, and which is fluid and soluble in 

 ether. This substance gradually passes out through 

 the stroma after the insect dies, and impregnates the 

 other tissues, but in a living insect the cornea has the 

 same kind of brilliancy as in the Vertebrate ; but as 

 soon as the insect dies the brilliancy rapidly fades, 

 and in a quarter of an hour it has become quite dull. 



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