88 C. F. MARSHALL. 



mesentery of the newt and from the bladder of the Salamander. In 

 both of these the fibrils in the muscle-cells are very evident, but the 

 intranuclear networks do not show at all distinctly, which is a most 

 unusual result in this mode of preparation. However, in prepara- 

 tions from the mesentery of the newt, made by Klein's method, the 

 intranuclear networks come out very distinctly in many fibres ; and 

 in one case I could trace the connection of the intranuclear network 

 with the fibrils of the cell. The longitudinal fibrils do not show so 

 well in these preparations as in those made by the gold method 

 (Figs. 12, 13). It thus appears that the vertebrate unstriped muscle 

 differs from all the invertebrate unstriped muscle that I have investi- 

 gated, in that the cells contain an intracellular network in the form 

 of longitudinal fibrils. This may perhaps represent a form of network 

 intermediate between the typical irregular network of other cells and 

 the highly modified network of the striped muscle-cell. 



From these investigations it appears that the peculiar intracellular 

 network of striped muscle is developed in all muscles which have to 

 perform rapid or regular movements. 



A brief review of the chief animals mentioned in the preceding 

 pages will make this clear. Commencing with the Actinia and the 

 Medusa, these are both highly organised Ccelenterates, but the 

 A ctinia is a sluggish animal, which exhibits slow and irregular move- 

 ments, while the Medusa propels itself through the water by rapid 

 and regular contractions of its disc. Now, in the Actinia we find no 

 striped muscle, but in the Medusa the network is present. In the 

 worms, such as the Leech and Earthworm, striped muscle is absent ; 

 these animals only performing comparatively sluggish movements- 

 In the Polyzoa the retractor muscles of the stomach, and in the 

 Rotifers the retractors of the trochal disc, perform rapid movements, 

 and have been described as striated transversely ; this is probably 

 due to the network, although, as stated above, I have so far been 

 unable to determine this myself. In the Mollusca the movements 

 are as a rule sluggish, and unstriped muscle is the prevailing type in 

 this group. But in the odontophore muscles of the Snails the move- 

 ments are more rapid, and in these we find the network developed. 

 Also in the hearts of these animals, which perform rapid and regular 

 contractions, I find the network present, at any rate in the case of 

 Patella. 



