ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 223 



knobbed after treatment with dilute solutions of cbromates or Miiller's 

 fluid. 



As touching tbe differences among ciliated cells, there is much 

 need for additional observations in so wide a field. The presence or 

 absence of fibres and pedicles, should such distinctions be confirmed, 

 offers a basis for primary divisions. " In the simplest cases the cilia 

 appear as direct prolongations of the protoplasm, or at least they 

 arise immediately from its superficial layer : thus is it with many of 

 the lowest unicellular organisms, as the zoospores of the lower plants, 

 the Flagellifera, the Ciliata, also in many embryonic as well as 

 definitive epithelial cells of the lower and higher Metazoa." Yet in 

 Vaucheria, as Strasburger has shown, the hyaline cortical protoplasm 

 of the zoospore contains minute radiating rods, each of which subtends 

 a cilium. In some ciliate Infusoria peculiar and more complex con- 

 ditions present themselves. In the higher animals with well- 

 specialized cilia we not only find, on comparing different cells, 

 diversities as to the number, size, and direction of the intracellular 

 fibres, but the chemical or optical behaviour of these fibres varies, 

 even where we meet with no obvious distinctions of texture. The 

 several arrangements of the cilia must also be considered. The cells 

 of lamellibranchs and a few vertebrates have hitherto been most 

 studied. Engelmann here figures and describes the two distinct 

 kinds of ciliated cells found in the intestinal epithelium of the 

 former ; he has further studied the cilia of the oral tentacles of 

 the mantle, and, especially, of the gills. He devotes part of his 

 memoir to extending and correcting the previous results of Posner. 

 Among rotifers, e. g, Brachionus, we see on a splendid scale an 

 arrangement of the cilia in parallel series, inclining obliquely to their 

 planes of vibration, similar to the more minute display of three 

 distinct systems on the respiratory cells of many bivalves. In 

 vertebrates the ciliary intracellular fibres are not easily seen. Never- 

 theless, Engelmann figures them as they appear in a cell from the 

 nasal lining of the frog. An exceedingly delicate striation could 

 also, after careful treatment, be detected in cells from the windpipe of 

 the rabbit. In many cases where no fibres have yet been discovered 

 they may eventually be proved to exist; their obstinate invisibility 

 being either the result of extreme fineness or of feeble differentiation 

 from the rest of the protoplasm. 



The physiological connection of the intracellular fibres with their 

 cilia is discussed by Engelmann in the last section of his present 

 memoir. That the fibres are not contractile, that they are still 

 quiescent while the cilia display active movements — seems indubit- 

 able. Stuart erroneously interpreted, as Engelmann here explains, 

 the apparent displacements of the nuclei in the ciliated cells of the 

 velum of larval Eolids, ascribing them to changes of position under- 

 gone by the fibres in question. Besides Stuart and Nussbaum (see 

 E, pp. 527-529), if we except a short and probably faulty account by 

 Bonnet, no other observer among the host of microscopists who have 

 seen cilia in action has noted contemporaneous intracellular move- 

 ments. In vain has Engelmann sought to detect such movements, or 



B 2 



