INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 547 



Microscopical Phenomena of Muscular Contraction.* — The micro- 

 scopical phenomena of muscular contraction have recently been studied 

 by Professor Th. W. Engelmann, of Utrecht. 



During the contraction of the transversely striated muscular fibres 

 there are produced, parallel with the changes of form of the muscular 

 elements, changes in the optical properties of the isotropic and aniso- 

 tropic layers. 



These changes are of an opposite nature in the two layers, the 

 isotropic stratum in its totality becoming more and the anisotropic 

 less refractile. 



In consequence, at a certain degree of contraction, the fibre seen 

 by ordinary light may appear homogeneous without appreciable 

 transverse strisB^the stage of homogeneity or of transition. 



If the contraction is carried further, the transverse strife corre- 

 sponding to the isotropic disks reappear. 



At any given phase of the process of contraction, consequently even 

 in the transition stage, the isotropic and anisotroj)ic substances may 

 be recognized by means of the polarizing Microscoj)e as well-defined 

 and regularly alternating layers. They do not, at the time of contrac- 

 tion, exchange their resp3ctive places in the muscular compartment 

 (i. e. that portion of the fibre included between the middle of the iso- 

 tropic layer and that of the next). 



The thickness of both layers decreases during contraction, that of 

 the isotropic layer much more rapidly than that of the anisotropic. 

 The total volume of each compartment does not vary sensibly during 

 contraction. 



The anisotropic layers increase in volume at the expense of the 

 isotropic, as during contraction liquid passes from the latter to the 

 former. 



Development of the Olfactory Nerve and Olfactory Organ of 

 Vertebrate s.f — In the course of an investigation into the development 

 of the cranial nerves of the chick, by Dr. A. Milnes Marshall, certain 

 facts came to light indicating that the olfactory nerve, instead of 

 being, as usually described, a structure diifering totally in its mode of 

 origin from all the other nerves in the body, in reality " exactly 

 corresponds in mode of development and in appearance with the other 

 cranial nerves, and with the posterior roots of the special nerves." J 

 A further paper by Dr. Marshall gives the results of further in- 

 vestigations on this point ; it deals also with some features in the 

 development of the vertebrate olfactory organ and with certain 

 questions of a more general nature affected by the conclusions arrived 

 at. The more important conclusions, as regards the development of 

 the olfactory nerve, are stated in the following propositions : — 



(a) The olfactory nerves do not arise from the cerebral hemi- 

 spheres, but from the single unpaired forebrain. 



* ' Arch. Neerland.,' xiii. (1S78) p. 437. 

 t ' Proc. Roy. Soc.,' xxviii. (1879) p. 324. 



X ' Proc. Roy. Soc.,' xxvi. p. 50, and ' Q. Journ. Micr. Sci.,' xviii. (1878) pp. 

 17-23. 



