716 SUMMARY OF CURRENT RESEARCHES RELATING TO 



about ; such nuclei are always small and vary in character, Schenk 

 has already noted the presence of non-nucleated cells in the ectoderm 

 when describing the process of fusion of the folds of the amnion. 



Scales, Feathers, and Hairs * — The idea largely taught to 

 students that scales, feathers, and hairs are identical in nature is 

 combatted by J. E. Jeffries. He considers the epiderm to be the 

 primitive skin, if not the true one, as it is formed long before the 

 corium, which is a late and very variable product of the mesoblast ; 

 and because all the organs of sense are formed from it. The epiderm 

 may be regarded as primitively consisting of a smooth mucous layer, 

 an epitrichial layer, and perhaps an intermediate layer of parenchy- 

 matous cells. In birds and mammals the outer layer is lost, and 

 never renewed, while the middle layer becomes thickened and subject 

 to various modifications, as drying, conversion into horn, &c., and 

 enters into the structure of all the appendages. Scales are moulted 

 and renewed, scuta are not. The toe-pads of birds may be seen to 

 pass over into scuta on the sides of the toes of many birds. Scuta 

 bear feathers as epidermal appendages — scales never do, thus pointing 

 to scuta, which have a mucous layer and outer horn coat with a 

 mesodermal core, as simple folds of the skin, not as appendages. 



The early stages of a feather and of a hair differ. The latter is 

 formed in a solid ingrowth of the epiderm, the latter from the 

 epiderm of a large papilla. A hair does not contain any of the 

 mucous cells, while a considerable portion of a feather consists of 

 them. The supposed homology between feathers and scales seems to 

 f.dl before the facts that the mucous layer is absent in the latter, and 

 that Studer has shown that the imagined scale-like nature of the 

 remiges of penguins is a fallacy. Mr. Jeffries avows his belief in the 

 distinct origin of the dermal appendages of the higher vertebrates, 

 and asserts that the nakedness of the Amphibia is a strong argument 

 against the identity of any of the avian appendages with those of 

 reptiles and mammals. 



Locomotion of Animals over smooth Vertical Surfaces.! — Dr. 

 H. Dewitz has extended his observations on this subject, at first 

 confined to insects, J to a variety of other forms, including some 

 Vertebrata. He finds that the same means, the exudation of a secre- 

 tion, are adopted in many cases, even vfhere sucking-disks are used. 

 Thus the leech can walk on a wire network, on which the disks could 

 not act by exhaustion of the air, and the secretion of the disks of 

 Piscicola has been examined by Leydig. A long series of animals is 

 enumerated from "Worms and Echinoderms to Apes among the 

 Mammalia, which are known with more or less certainty to use similar 

 means for climbing. 



The tree-frog (Hyla) maintains its hold as firmly within the 

 exhausted receiver of an air-pump as in the open air, and in fact a 

 piece of glass passed over the balls of the tips of the toes shows clear 



» Proc. Bost. Soc. Nat. Hist. Of. Amer. Natural., xviii. (1884) p. 640. 

 t Pfliiger's Arch, gesanunt. Physiol., xxxiii. (1884) pp. 440-81 (3 pis.). See 

 also infra, Insecta. 



X See this Journal, iii. (1883) p. 363. 



