396 SUMMARY OF CURRENT RKSEARCHES RELATING TO 



Histogenesis of Thymus in Amphibia.* — A. Maximow has studied 

 this in Siredon pisciformis and in the frog. Hammar's view is correct 

 as to the epithelial nature of the reticulum of the cortex and of the 

 large cells of the medulla, and as to the lymphocyte nature of the small 

 cortical cells which are derived from the mesenchyme. While the thymus 

 is still a simple epithelial primordium, migrants from the mesenchyme 

 invade it. The epithelial cells remain passive, the mesenchyme cells 

 grow and multiply, and new invaders throng in. The epithelium be- 

 comes a reticulum ; the cells grow, but not so much as the lympho- 

 cytes ; the reticulum is firmer on the surface of the organ. As the 

 thymus grows, protul)erances arise on the surface, and between these 

 connective-tissue septa and blood-vessels grow in. When the blood- 

 vessels get into the depth of the organ the medullary substance is formed 

 in their vicinity, the epithehal cells undergoing Jaypertrophy and the 

 lymphocytes removing themselves. The lymphocytes in the periphery 

 give rise to smaller lymphocytes, the thymus being in this sense a blood- 

 forming organ. The main peculiarity of the organ is that two kinds 

 of tissue, epithelium and mesenchyme, usually so distinct, become very 

 intimately associated — but the same sort of thing occurs elsewhere, e.g. 

 in the tonsils and in the bursa Faliricii of birds. 



Pigment-cells of Fishes.f— Karl von Frisch has studied these in 

 the minnow {Phoxinus Isevls), with particular reference to the influence 

 of temperature. On the lilack pigment-cells of the skin a local aj^plica- 

 tion of heat induces expansion, and cold, contraction. This effect is 

 restricted to the locality of the stimulus ; it is independent of the circula- 

 tion ; it is not due to any spinal cord reflex. It is uncertain whether the 

 effect is due to direct action on the pigment-cells or to a reflex through 

 the sympathetic system. 



Lymphoid Structure on Brain of Lepidosteus.| — Asa C. Chandler 

 describes a large lymphoid gland-like structure overlying the myelen- 

 cephalon of Lepidosteus, and closely associated with the pial covering of 

 the fourth ventricle. This " myelencephalic gland " is present in both 

 L. osseiis and L.platysto^nus. A reticulum of connective-tissue underlies 

 the whole, and in this a number of other histological elements are netted. 

 The most characteristic feature is the abundance of cells containing 

 masses of granules staining deeply with eosin, of unknown nature, and 

 resembling melanin granules in size and form. There is some evidence 

 that these granules pass into the blood and disintegrate. 



Structure of Salmon Scales. § — Philippa C. Esdaile has made an 

 intensive study of the scales of three specimens of Salmo salar. A great 

 variation in the number of annuli and in the lengths of the scales taken 

 from different parts of the same fish is clearly indicated. This was 

 found on each of the three fishes, but the results obtained seem to be in 

 no way correlated. It is to be noticed that in the three fishes ex- 

 amined the number of annuli in each " peronidium " Ca term for an 



* Arch. Mikr. Anat., Ixxix. (1912) Heft 4, Abt. i., pp. 560-611 (3 pis.). 



t Biol. Ceutralbl., xxxi. (1911) pp. 236-48 (3 figs.). 



X Univ. California Publications (Zool.) ix. (1911) pp. 85-104 (3 pis.). 



§ Mem. and Proc. Manchester Lit. Phil. Soo., Ivi. (1912) pp. 1-20 (6 pis.). 



