ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 381 



Granula and Mitochondria in Somatic Cells.* — Julius Arnold 

 defeuds bis theory of the importance of the granula in the metabolism 

 of the cell. The formed elements of the plasma, whether they be 

 granules (plasmosomes or granula), or simple tilamentar granules (blas- 

 tomites), or mitochondria, have always an important role in the processes 

 of metabolism. 



Hsematopoiesis in Chelonia.t— H. E. Jordan and J. C. Flippin find 

 that during the first half of embryonic life the area vasculosa of the 

 yolk-sac is the chief ha3matopoietic organ. During the second half 

 the bone marrow mainly subserves the haBmatopoietic function. After 

 the very earliest stages (8 mm. in length) the body mesenchyme 

 generally aids in the ha^matopoiesis. This process is especially active 

 in the head-region, about the cerebral vesicles, in the mesenchyme sur- 

 rounding the gut and the larger blood-vessels, in the dermis of older 

 forms, and in the interstitial membrane of the mesonephros. In the 

 latter the process is predominantly ieucopoietic, producing lymphocytes, 

 thrombocytes, and eosinophile granulocytes. Still another source of 

 lymphocytes is the endothelium of the blood-vessels, more particularly 

 of the smaller ones. The sinusoids of the hver, heart, and mesonephros 

 offer favourable locations, due to slowness of curi'ent, for cell prolifera- 

 tion. 



The histogenesis of the blood is similar in the blood islands of the 

 yolk-sac, in the bone marrow, and in the body mesenchyme. A large 

 amoeboid cell — which traces its ancestry back to undifferentiated mesen- 

 chyme — with basophile cytoplasm and vesicular nucleus with nucleolus, 

 in short, the primitive blood-cell or ha^mogonium, differentiates intra- 

 vascularly into erythrocytes, extra-vascularly into granulocytes. The 

 undifferentiated proliferation products persist intra- and extra-vascularly 

 as lymphocytes. These lymphocytes remain as potential h^emogonia. 

 The endothelium of blood-vessels produces internally small lymphocytes 

 and thrombocytes : externally, lymphocytes and granulocytes. 



The various types of blood-elements in the turtle trace their ancestry 

 back to a common progenitor, the " lymphocyte," whether produced in 

 yolk-sac, body mesenchyme, or bone marrow. The blood-platelets of 

 Mammals — detached cytoplasmic fragments of mega-karyocytes — corre- 

 spond to the spindle-cells of other Vertebrates. These spindle-cells 

 stand in direct genetic relationship to the endothelium of blood-vessels 

 and to the small leucocytes. 



Innervation of Dentine. J — J. Howard Mummery has shown that 

 the dentine is richly supplied with nerves from the pulp, which do not 

 terminate, as has been hitherto generally supposed, at the inner margin 

 ■of the dentine, but enter the tubules of that tissue and traverse them 

 to their peripheral terminations at the enamel and cementum margin. 

 From the marginal plexus of the pulp, neurofibrils pass in great abund- 

 ance into the dentine tubules. In the majority of cases there appear to 

 be two fibres in each tubule. 



* Anat. Anzieg, xliii. (1913) pp. 432-62. 



t Folia HBematologica, xv. (1913) pp. 1-24 (1 pL). 



I Proc. R. Soc, Series B, Ixxxv. (1912) pp. 79-80. 



