186 ARTHUR E. SHIPLEY. 



remaining clear and staining very slightly. The nucleus lies 

 in this medullary portion. The longitudinal thickenings occur 

 at intervals, so that in transverse section the cortex of the cell 

 appears beaded ; these fine fibrillae stain fairly well so that 

 they can easily be distinguished from the medulla. The flat- 

 tening of the cell goes on until the cell occupies the whole 

 space between two myotomes, not only longitudinally but also 

 transversely (fig. 25). The original nucleus of each cell 

 divides into two or three, so that in each of these plates of 

 muscle-substance two or three nuclei can be seen and an occa- 

 sional yolk granule, which is, however, soon absorbed. In 

 addition to the longitudinal striation caused by the thread-like 

 thickenings in the cortex, a transverse striation appears. Each 

 plate of muscle-substance remains in this condition, with a 

 clear unstained medulla containing two or three deeply stained, 

 large, flat, oval nuclei (fig. 18), with a well-marked nucleolus; 

 enclosed by a cortex, for about two weeks after hatching. The 

 cortex consists chiefly of its dorsal and ventral walls, and each 

 of these is thickened at regular intervals by the above-men- 

 tioned fibrillae. Each fibrilla runs the whole length of the 

 myomere and is inserted into the intermuscular septa behind 

 and in front. About a fortnight after the young Ammocoete 

 is hatched, the substance of the fibrillge increases at the 

 expense of the medullary part, and this goes on until each 

 plate of muscle-substance consists exclusively of fibrillar sub- 

 stance. The nuclei have increased in number, but instead 

 of lying loose in medulla they become squeezed in be- 

 tween the fibrillse, lose their regular shape and can only 

 be recognised as small flattened bodies which stain deeply. 

 The whole plate of muscle-substance now consists of fibrillar 

 substance which stains uniformly with here and there a more 

 deeply stained nucleus (fig. 29). The whole appears homo- 

 geneous, the fibrillae cannot as a rule be recognised, though in 

 some cases they are seen in transverse section as dots. Each 

 " Kastchen " now resembles fundamentally the muscle-plate of 

 the adult Lamprey ; and it will be noticed that each is a deve- 

 lopment of what was a single cell. 



