GILL DEVELOPMENT IN MYTILUS. 



6 7 



ranged accurately in serial order. In Fig. 4 are seen isolated 

 portions from the posterior end of the ctenidium. The speci- 

 mens figured are selected from a large number of dissections, and 

 are taken from several individuals of slighty varying size. Hence 

 the discrepancy in the size of the figures, as all are equally 

 magnified. All the stages figured have been observed again and 

 again. 



From the figures it is clearly seen that we are dealing with 

 something very different from the comparatively long and slender 



Fig. 2. Posterior tip of ctenidium of Mytihis of 3.0 mm. length, spread out, and 

 viewed from dorsal side. Inner gill below and to right. Magnification 170. 



rod-like papillae observed in younger specimens. The youngest 

 anlagen are rather transverse folds or ridges on the dorsal and 

 posterior sides of the gill axis. At first these ridges are compara- 

 tively short and of uniformly convex contour (Figs. 4, A ; 2, a ; 

 3, a). Then the ridge elongates and becomes flat topped (Figs. 

 2 > ^ 5 3» ^) J an d soon a process of differential growth or constric- 

 tion leads to a notching of the ridge near its middle (Figs. 2, c ; 

 3, c ; 4, B). This notch divides the originally simple anlage into 

 two slightly unequal parts, the larger belonging to the outer gill 

 and the smaller to the inner. These two parts grow rapidly 



