SUMMARY OF CURRENT RESEARCHES. 29 



developing tissues, and the maturation division a special case of this, and 

 the loss of the nucleus in the red blood -corpuscles of Mammals a terminus 

 in the series ? 



He maintains that this question should be answered in the affirma- 

 tive. In the differentiation of blood-corpuscles there is an extrusion of 

 chromatin from the nucleus, which is comparable to polar body formation. 

 Many tissue-cells show, as they differentiate, processes comparable to 

 chromatin-reduction in the maturation of ova or of spermatozoa. The 

 author illustrates his theory with reference to the histogenesis of nerve- 

 cells and the history of the yolk in Vertebrates. There is chromatin- 

 diminution in diverse tissues and stages ; it is a general phenomenon. 

 At the one extreme we have bacteria which are almost wholly nucleus ; 

 at the other extreme we have Mammalian red blood-corpuscles, where 

 the nucleus disappears. 



Size of Cells and Body-growth.* — H. Plenk suggests that in lower 

 animals of small size, e.g. in some Nematodes and Rotifers, the increase 

 in the dimensions of cells plays a very important part in the growth of 

 the body. Cell-multiplication, however, is the main factor of growth in 

 those animals whose definitive size is very different from that of the 

 embryo. But even here an increase in the dimensions of cells has its 

 role. The author shows this increase (1) in permanent elements, like 

 ganglion-cells, muscle-cells, lens-libres, which lose very early their power 

 of division ; (2) in the cells of some animals with small eggs, like lam- 

 prey and salamander ; (3) in regard to peculiarly large cells. The rapid 

 succession of cell-divisions seems to work against increase in size. Each 

 stage in development has its specific size of cell. The nucleo-cytoplasmic 

 relation in embryonic cells is different from that in the adult. This 

 depends partly on the fact that the nucleus has already attained its 

 definitive size while the cytoplasm has still to grow. In some cases the 

 nucleus may be a little smaller than in the adult ; in some cases larger. 

 It is noteworthy that in ganglion cells the nucleus grows along with the 

 cytoplasm. 



Displacement of Optic Lobes in Fowl's Brain. -f — K. K. Oliver 

 presents a series of drawings to illustrate the manner in which the optic 

 lobes form and change their position. The earliest drawing shows the 

 mesencephalon as a single median structure on the dorsal surface, and 

 the subsequent ones its division into a bi-lobed structure, and its dis- 

 placement caused by the development and growth both of the lobes 

 themselves and of the adjacent parts. The later drawings depict the 

 lateral and ventral course of the optic lobes and their final attainment 

 of the adult latero- ventral position. 



Subdivision of Spinal Canal in Lumbar Region of Chick Em- 

 bryos.l — Helen Kelsey reports that a division of the central canal of 

 the spinal cord occurs invariably in the lumbar region at a certain stage 

 (•48-58 hours' incubation) of development. In most cases three canal 

 were distinctly seen ; in several there were four, and in one instance an 



* Arbeit. Zool. Inst. Univ. Wien. xix. (1911) pp. 247-88 (2 pis. and 4 figs.). 



t Proc. R. Soc. Victoria, xxiv. (1911) pp. 78-91 (11 pis.). 



X Proc. R. Soc. Victoria, xxiv. (1911) pp. 152-G (1 pi. and i figs.). 



s 



