22 



WHEELER. [Vol. VIII. 



in their growth before they unite. The anterior and posterior 

 edges, however, continue to advance without interruption, so 

 that the disk if spread out on a plane surface would in its suc- 

 cessive stages represent a series of ellipses with constant short 

 axis but continually increasing longitudinal axis. In this 

 manner the disk grows towards either pole while envelop- 

 ing the ^gg laterally. The edges of the organ continue to 

 approximate on the dorsal surface but stop growing just before 

 they meet. Hence, when the egg is viewed from the dorsal 

 surface a long, narrow slit is seen extending nearly its entire 

 length and separating the dorsal edges of the organ. It is not 

 till the anterior and posterior edges have nearly or quite 

 reached their respective poles that this slit closes with the 

 fusion of the edges of the organ. The raphe is at first so 

 weak that the edges may be broken apart by slight pressure 

 with the needles, but it soon becomes permanent and the Qgg 

 is now completely enveloped by two further membranes — the 

 inner and outer indusia. Before the fusion of these two mem- 

 branes the amnion of the embryo was in contact with the 

 serosa but now that the edges of the indusia have worked 

 their way in between the serosa and amnion, the latter comes 

 to lie in contact with the inner indusium. Henceforth the 

 serosa is excluded from taking any part in the development 

 of the embryo; both its position and function are now usurped 

 by the inner indusium. 



One is enabled to follow the different stages in the progress 

 of the indusium, from its disk-like condition on the ventral 

 yolk to the complete union of its dorsad-growing edges, by 

 means of a peculiar secretion of its inner layer. This is a 

 brownish or blackish granular substance, probably some urate, 

 which appears to be secreted by all the cells of the inner 

 indusium and which gives the organ the appearance of a large 

 brown blotch in a stage a little older than E. At first pale and 

 hardly perceptible, this spot gradually deepens in color till its 

 advancing edges become distinctly outlined on the underly- 

 ing yolk. A clear idea of the closure of the edges may be 

 obtained from Fig. Ill, A-C. The dark granular secretion 

 is shown in Fig. 6 at envl. 



