ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 21 



from the endodermic branchial primordia. As to the adult organ, 

 there is but one characteristic element, the small thymus cell. The 

 others are secondary mesodermic importations — connective cells, myo- 

 epithelioid cells (HassalFs corpuscles), granular cells and leucocytes, 

 vascular epithelium, and blood corpuscles. The seasonal and definite 

 involution of the thymus is carefully discussed. As to function, the 

 Reptilian thymus shows a seasonal alteration of great activity and 

 repose, the former gradually waning. During activity the thymus is 

 alternately the seat of vascular neo-formation and retrogression. The 

 small thymus cells are actively divided, but the organ is not lympho- 

 poietic, or leucopoietic, or erythropoietic. Nor is there any specialised 

 secretion. Its activity is seen in changes in the number of small 

 thymus cells. The myo-epithelioid cells and Hassall's corpuscles are 

 metaplasmic forms of connective cells produced under the action of the 

 small thymus cells. 



Phosphorescent Organs of Fishes.* — 0. Steche describes the 

 phosphorescent organs of two surface fishes, Anomalops Icataptron and 

 PhotobJepharon palpebratus, occurring in the Malay Archipelago. He 

 was able to observe them frequently under natural conditions, and to 

 keep them alive in captivity for some time. Morphologically, the 

 known luminous organs of fishes may be arranged in two series. The 

 first series consists of acinous glands ; it begins with open forms, but, as 

 specialisation proceeds, these may lose their ducts and become round 

 sacs with no lumen, and with hardly any indication that they are made 

 up of glandular tubules. The organs of this series usually occur on the 

 head or appendages, so that their light illumines the animal's field of 

 vision. Those of the first series are much more richly supplied with 

 blood and nerves than those of the second. The most important con- 

 stituent of the second series is also glandular cells, but these are not 

 disposed to form typical glands. They form an accumulation of 

 individual cells with no lumen, except in Gonostomidae. They are 

 derived from differentiated epidermal cells, which have become united 

 and transferred to the cutis. It is still uncertain whether the glandular 

 cells are the starting-point of this development, or whether the organs 

 are derived from sensory papillae. 



The organs of the second series are smaller but much more 

 numerous than those of the first, and they are sparingly supplied with 

 blood and nerves. They contain lenticular cells, and occasionally 

 gelatinous matrix, differentiations which never occur in the first group. 

 The lenticular cells occur even in very primitive organs. Even the 

 simple epidermoidal organs show a characteristic orientation to the 

 surface of the body in accordance with their position on the trunk, in 

 the same way as the most highly developed representative of the group. 



Functionally, the two groups also differ markedly from one another. 

 In the first group the luminosity is usually extra-cellular, arising within 

 the cells only in the most extremely modified organs. It is, as far as 

 observations have shown, constant and very intensive. No kind of 

 stimulation has any effect on it. The organs of the second group 



• Zeitschr. wiss. Zool., xciii. (1909) pp. 345-405 (3 pis. and 5 figs.). 



