ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 179 



•of the structure of the adult organ, and its histogenesis, he gives full 

 details of the conditions in the different forms. Other sections deal 

 with myoid cells, age-involution, and accidental involution. Each 

 section begins with a consideration of the literature relating to it. The 

 general results of the investigation, which, the author considers, may 

 throw light on some cardinal points in the general morphology and 

 physiology of the organ, are summarised as follows. The fact that the 

 Teleostean thymus is, not only in its early stages but throughout life, 

 in the greater number of the forms examined, an integral part of the 

 surface-epithelium of the gill-cavity, demonstrates the epithelial origin 

 and constitution of the thymus reticulum, both as regards core and 

 cortex. The downward growth found in some forms, and the relatively 

 late separation from the epithelium, which was found in one form 

 (Cyprinus), is of interest as showing a transition to the conditions 

 found in other Vertebrates. The freedom of the thymus-rudiment from 

 vessels, and the simple form of its boundary adjacent to the connective 

 tissue, make' it easy to determine the passing of numerous lymphocytes 

 through the boundary-surface during the differentiation of the organ. 

 Exact measurements and calculations showed that this passage of the 

 lymphocytes is to be interpreted as an immigration, not an emigration. 

 The theory of the autocthonous origin of the thymus lymphocytes is 

 apparently irreconcilable with the state of things found in the Teleostean 

 thymus. 



The occasional occurrence of myoid cells in the placoid Teleostean 

 thymus, before it has been invaded by blood-vessels or connective-tissue, 

 excludes the possibility that these cells have been either brought in or 

 shut in. The occurrence of transverse striped fibrillas in the typical 

 reticulum cells also points to an autocthonous origin of the myoid cells. 



An involution of the thymus takes place in Teleosteans, under 

 essentially the same forms as it occurs in the higher Vertebrates. It is 

 probable that in this case also there is a relation between sexual maturity 

 and age-involution. Withdrawal of nourishment gives rise to accidental 

 involution of the thymus in Teleosteans in the same way as in other 

 Vertebrates. 



Eels and their Migrations.* — W. L. Bishop gives an account of 

 some observations and experiments on eels, with special reference to the 

 season of their migration, and the length of time they can remain out 

 of water. He was led to these observations by the fact that in the 

 waterworks of Dartmouth, Halifax, eels caused considerable trouble by 

 continually getting into the water-mains, and blocking up the service- 

 pipes to the town. Men were constantly employed in digging up these 

 pipes in September and October, and from thirty to fifty eels were 

 taken from the pipes each year in those months. The pipes to the town 

 were the only outflow from the lake at that season. A trap was arranged 

 at the gate-house to the water-supply, and a varying number of eels 

 were taken every day, the highest figure reached being 308, on August 26. 

 All the relatively high numbers occurred after heavy rainfall. 



The eels taken in May and June were invariably of small size, and 



* Proc. and Trans. Nova Scotia Inst. Sci., xi. (1906) pp. 640-50. 



