ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 565 



literature and a full bibliography. His material included various 

 Teleosts, especially species of Salmo, and the chick, but to the ordinary 

 methods of sectioning he added dissection xinder a simple Microscope, 

 and examination of living embryos. In Teleosts he finds that the en- 

 cephalon is divided into eleven primary segments, which are indicated 

 both externally and internally, and which precede the familiar division 

 into fore-brain, mid-brain, and hind-brain. These latter are not simple 

 divisions, but result from a fusion of the original segments in the 

 following way : — 3 segments go to the fore-brain, 2 to the mid-brain, 

 and 6 to the hind-brain. In the last case, 1 segment constitutes the cere- 

 bellum, and the remaining 5 constitute the medulla. In the chick the 

 same 11 segments are represented, but in the anterior region they are 

 very transient. In consequence of this, and of the fact that in the 

 region of the medulla the segments persist for some time, the segments 

 of the medulla have been erroneously homologised with secondary ex- 

 pansions of mid-brain and fore-brain, instead of with the primary 

 segments of these regions. If this error be avoided, there is no diffi- 

 culty in showing that the primary encephalic segments of the chick are 

 in all essentials identical with those of the trout, and have the same 

 relations to the adult parts. 



b. Histolog-y. 



Cell-Structure.* — Prof. G. Ballowitz has studied the epithelium of 

 the membrana elastica posterior of the eye as a good subject for minute 

 histological study. He finds that the cells contain a giant sphere which 

 modifies in a remarkable way the form of the excentric nucleus, and is 

 more distinct in old than in young animals. Within the sphere lies 

 the microcentrum, usually made up of two central corpuscles. These 

 appearances resemble in all respects the conditions previously found by 

 the author in Salpa ; and he believes that only technical difficulties stand 

 in the way of the demonstration of similar conditions in other cells. The 

 most important distinction between the corneal epithelium and the epi- 

 thelium of Salpa is as regards the minute structure of the sphere. In 

 the corneal cells the sphere consists of a basket-work of threads, the 

 meshes being of unequal size, and the spheres of the different cells 

 showing extraordinary variability. In spite of this variability and the 

 differences from the spheres of other cells, the author is of opinion that 

 the spheres are not peculiar structures, but true cell-spheres. They 

 show no " system of organic radii," and the peculiar basket-work remains 

 unaltered during the changes of the nucleus ; and therefore the author 

 believes that they disprove Heidenhain's " law of tension,' 1 and all the 

 theories involved in that law. The giant spheres are to be regarded as 

 special cell-organs ; but as in adult animals the cells of the membrana 

 elastica posterior do not undergo division, the relation of the spheres to 

 the mitotic process could not be studied. The very extensive modifica- 

 tions of the form of the nucleus which occur under the influence of the 

 sphere both in the epithelium of Salpa and in that of the eye, are to be 

 explained as the result of the great thinness of the flattened cells, and 

 the consequent internal pressure. 



* Arch. Mikr. Anat., lvi. (1900) pp. 230-90 (3 pis. and 49 figs.). 



