i895. NOTES AND COMMENTS. 379 



Devonian periods, but apparentl}' never represented later, is that of 

 the singular group of Pteraspidians {see Natural Science, October, 

 1892). The greater part of the thickness of the plates of these 

 organisms is formed by delicate structureless lamellae or laminae, con- 

 centric with the exposed faces, exhibiting no minute cavities for pro- 

 toplasm-cells, but surrounding and enclosing great vascular spaces. 

 The outer layer alone consists of ridges, through which delicate 

 capillar}' canals radiate from a central soft tissue of which no trace 

 is fossilised. 



Reconstructions from Series of Sections. 



An elaborate and technical paper by Mr. H. B. Pollard (pub- 

 lished in the eighth volume of Spengel's Zoologische Jahvhncher) con- 

 tains the results of work based upon a method little practised or, 

 indeed, known in England, though long since described in Natural 

 Science.^ Mr. Pollard has been investigating the anatomy and 

 development of the head in siluroids and other fishes, for some years, 

 and he has now perfected, at the Owens College, the method of Born 

 which he learned in Germany. When a complex structure like the 

 head of a young fish has to be investigated by means of series of 

 sections, it is, as everyone knows who has worked with continuous 

 series, very difficult to construct from the consecutive drawings a solid 

 image in the mind. Mr. Pollard built up models in the following 

 way : The heads of the young fish were decalcified, and double- 

 stained with alum carmine and bleu de lyon. They were then cut in 

 a series, each section being of known thickness. Next, drawings of 

 the sections were made with a camera, and the drawings were trans- 

 ferred to plates of wax. The thickness of each plate v/as carefully 

 made the same multiple of the thickness of the section that the 

 camera drawing was of the area of the section. The plates of wax 

 "were then cut out following the drawing, so that a series of wax repre- 

 sentations of the series of sections was obtained. These were too 

 large and too brittle to be handled conveniently, and the special 

 improvement of Mr. Pollard upon Born's method was that he electro- 

 plated his wax slices. The plated slices were then fixed together in 

 their order, and the resulting model was, within slight limits of error, 

 a faithful copy of the original head. In his paper Mr. Pollard gives a 

 number of drawings from the models. 



The Heart in Birds and Reptiles. 



In a recent number of the Pyoceedings of the Zoological Society 

 (part iii., 1895), ^lessrs. F. E. Beddard and Chalmers Mitchell 

 describe the heart of the alligator, giving a number of useful figures. 

 They refer at some length to the interesting morphological question of 

 the homology of the valve in the right ventricle of the bird's heart. 



Vol. iii., p. 340, in " Natural Science at the Chicago Exhibition." 



