ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 



865 



the amphibian and elasmobrancli ova furnishes nothing incompatible 

 with this fact. . 



Agreement is expressed with His and Eauber in the conclnsions as 

 to the nature of the process by which the embryonic ring becomes 

 converted into the embryo ; the median plane of the embryo appears 

 almost certainly to coincide with the first plane of cleavage. The 

 authors think with Balfour that the neural surface is identical through- 

 out the Metazoa. A full memoir is promised. 



Cell-Division, the Relation of its Direction to Gravity and other 

 Forces.*— E. Pfliiger has recently extended his observations on the 

 influence of various forces on the direction in which cells divide, with 

 especial reference to the ova of Batrachia. An ovum may be re- 

 garded as a bladder filled with a fluid not uniform in consistency. 

 The granular portions tend, under the influence of gravity, to sink, 

 whilst the specifically lighter nucleus floats in the lighter fluid towards 

 the upper surface. The form of the ovum, acted upon by gravity 

 alone, will be somewhat flattened vertically, the shortest (or vertical) 

 diameter being the " symmetric axis " which does not coincide with 

 the primary (" non-asymmetric ") axis of the ovum. The karyokinetic 

 figures of the nucleus consist of a rearrangement of the nuclear 

 network in a direction at right angles to the symmetric axis. 

 Pfliiger concludes that this rearrangement is in the direction of least 

 resislance, as being entirely in the lighter non-granular portion of the 

 cell-contents. Vertical rearrangement would have to thrust aside 

 the heavier granular portion lying inferiorly. 



To verify this view, Pfliiger compressed ova between two glass 

 plates so as to assume the form of strongly flattened ellipsoids. In 

 80 to 90 per cent, such ova divided after showing vertical karyo- 

 kinetic spindles parallel to the surface of the glass. This result 

 follows because the thinner and lighter fluid at the top has a greater 

 vertical extension and the influence of gravity is no longer exerted at 

 a disadvantage. In fact, the direction of least resistance is now 

 vertical. 



The conclusion is that under the influence of gravity, pressure, or 

 other forces, the dividing cell rearranges its elements preparatory to 

 division in that direction in which it meets with least resistance. 

 On the other hand, W. Roux f and G. Born | have separately come to 

 the conclusion that Pfliiger's view is not justified by facts. lioux, 

 working with frogs' eggs, used a centrifugal apparatus and found 

 that, whether the centrifugal force were stronger or weaker than the 

 force of gravity, cell-division took place uniformly in the same 

 direction, invagination occurring always on the border of the (heavier) 

 white hemisphere and of the (lighter) dark hemisphere. Consequently, 

 " the force of gravity is not indispensably necessary to normal 

 development, and it has no necessary directive influence on tho 



♦ rfliigcr's Arch. f. <1. gosammt. Physiol., xxxiv. (1884) pp. C07-10. Of. 

 Naturfbrbclicr, xvii. (1881) r'P- 372-4. 



t BroHlaucr Aerztl. Zeitbchr., 1884, No. C. 

 t Ibid., No. 8. 



