ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 27 



withiu a muscle-fibre suggests that the fibrils are firm elastic threads, 

 which were thrust to the side by the worm's movements. If the 

 contractile substance is fluid, it should be affected by gravity, unless the 

 capillary forces are sufficient to antagonise this. An ingenious experi- 

 ment with a centrifugal machine showed that rotations of 1200-1400 

 per minute did not affect the distribution of the contractile substance, 

 though the force was some 400 times greater than that of gravity. 

 When a fresh fibre is cut with a sharp knife, nothing exudes, and this is 

 surely against the assumption of a fluid contractile substance. But the 

 view that the fibrils are firm elastic threads also present difficulties, 

 especially as to the formation of the transverse disks. Hurthle asks 

 consideration for the idea of functional transverse connections, which 

 appear in certain functional conditions of the muscle and disappear in 

 others. In the process of contraction there may be a strengthening of 

 the framework. If the muscle is regarded as an elastic band, its 

 modulus of elasticity is much lower than occurs in any inanimate body. 

 With Briicke, we must still say " Der Aggregatzustand des lebenden 

 Muskels ist ein Geheimnis eigentlimlicher Art." 



Tetrads in Somatic Cells.* — Paolo della Yalle has found distinct 

 " tetrads " in various somatic cells of larval salamanders and in the root 

 of the pea. In the metaphase of some mitoses, among the other 

 chromosomes, there are typical tetrads with granular or elongated 

 elements. The author regards the occurrence of tetrads as quite 

 accidental. It is seen whenever a chromosome, with a transversal 

 splitting, divides at the metaphase and the two halves are not separated 

 towards the two poles. It has nothing to do with the re-union of 

 homologous chromosomes. 



Secretion of Mammary Glands.f — F. Bertkau maintains that the 

 formation of milk is purely a secretory process, and that there is no 

 necrobiosis of any kind on the part of the secretory epithelium. Those 

 who have described necrobiotic processes have been deceived by imperfect 

 technique. The cells, like the muscle-cells of sweat glands, between 

 the membrana propria and the epithelium of the glandular alveoli, are 

 true smooth muscle-cells. 



Vindication of the Neuron Theory 4 — S. R. Cajal states the case 

 for the neuron theory of His and Forel. He brings forward a series of 

 facts, based on the study of nerve-regeneration, which support the histo- 

 genetic theory of His and Kupffer. He follows that with a statement 

 of the arguments based on embryonic neurogenesis. The result is a 

 convincing vindication of the neuron theory. The illustrations of the 

 paper are remarkably fine. 



Valves in the Veins of a Frog.§ — E. Suchard finds that there are 

 numerous sigmoid valves in the veins of Rana escuUnta. They are 

 comparable to those of Mammals, and are perfectly developed. They 



* Atti R. Accad. Sci. Napoli, xiii. (1907) 39 pp. (1 pi.). 



+ Anat. Anzeig., xxx. (1907) pp. 161-80 (7 figs.). 



j Tom. cit., pp. 113-44 (24 figs.). 



§ C.R. Soc. Biol. Pans, lxii. (1907) pp. 452-3. 



