ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 445 



Histogenesis of Muscle.* — J. Mlodowska has studied the develop- 

 ment of skeletal muscle in embryos of fowl, mouse, rabbit and pig. A 

 muscle-fibre is equivalent to several cells fused in a syncytium ; it may be 

 derived from cells which have belonged to several myomeres ; the myo- 

 blasts of adjacent myomeres coalesce by means of ever-broadening 

 bridges of plasma, on which fibrils grow ; the disappearance of the 

 metamerism is helped by mesenchyme cells, which arrange themselves in 

 rows and unite with the bridge by numerous processes. The contractile 

 fibrils are either the product of a single myoblast (in this case they 

 grow independently into the other myoblasts which compose the syncy- 

 tium), or they may owe their origin to several myoblasts (in this case 

 they subsequently unite by their ends to form a single long fibril). A 

 fibril may be the product of cells belonging to several myomeres. The 

 fibril formed in one myomere may grow independently into the cell- 

 territory of adjacent myomeres, the plasmic bridges previously formed 

 serving as transitions. Fibrils have a certain individuality — of growing 

 beyond their origin, of increasing in length and thickness, and of mul- 

 tiplying by splitting. Organogenetic processes lead to over-production 

 of embryonic muscular tissue : part undergoes degeneration, and part 

 becomes connective. Increase in the number of muscle fibres is due 

 not only to longitudinal splitting of fibres, but also to the co-operation 

 of mesenchyme cells which form chains. 



c. General. 



Scales of Fishes.f — E. S. Goodrich has studied the different types 

 of scale in fishes, with special reference to their use in classification. 



1. The placoid scale of Selachii and Holocephali begins as a cone 

 of dentine deposited by mesoblastic scleroblastic cells below the epi- 

 dermis, in continuity with the basement membrane. A basal plate 

 may be present in the form of a direct extension inwards of the cone, 

 never as a separate element which becomes fused on to it secondarily. 

 Both the cone and the plate are composed of dentine or some allied 

 substance, never of true bone ; the cone may pierce the epidermis when 

 fully grown. The scales and plates of Heterostraci have been evolved 

 by the combination of a covering of separate isolated denticles and an 

 underlying plate. 



2. The cosmoid scale, e.g. of Megalichthys, consists of a basal layer of 

 parallel bony laminas of " isopedine," over which is a bony zone with 

 large vascular spaces. These canals combine near the surface in a 

 more regular horizontal system, with vertical canals reaching the outer 

 surface. Below the surface the canals expand into conical chambers, 

 between which pass upwards another set of vertical canals, ending 

 above in pulp-cavities. From these radiate a multitude of canaliculi. 

 Williamson gave the name of "cosmine" to a peculiar dentine-like sub- 

 stance in " ganoid " scales, and Goodrich uses the term cosmoid for 

 scales with a tissue with canaliculi like those of dentine. The cosmoid 

 scale has an outer layer of dentine-like substance, a middle bony layer 



* Bull. Internat. Acad. Sci. Cracovie, 1908, pp. 145-71 (2 pis.). 

 t Proc. Zool. Soc, 1907, pp. 751-74 (4 pis., 9 figs.). 



