1 8 WHEELER. [Vol. VIII. 



By the time the folds have closed over the indusium the 

 abdomen of the embryo has sunk into the yolk to a con- 

 siderable extent, presenting in surface view the appearance 

 of Fig. 4. The organ seems to undergo no further change till 

 the embryo has almost left the ventral face of the egg. Then, 

 as we have seen, it begins to increase by spreading. An early 

 stage in this process is shown in section in Fig. 20. No change 

 is perceptible in the serosa, which is now independent of the 

 organ ; the outer indusium {a7n'^) is much attenuated, as may 

 be seen by comparison with Fig. 19. Its cells have assumed 

 the same shape and dimensions as those of the superjacent 

 serosa ; only along the edges of the disk, where the outer 

 becomes continous with the inner indusium, or body of the 

 organ, do the cells still retain their original shapes. In 

 the body of the organ the cells are arranged in two irreg- 

 ular rows, whereas in the previous stage (Fig. 20) there were 

 three. This diminution in the number of cell-rows is the 

 result of horizontal spreading, a process which also accounts 

 for the stretching of the outer indusium as indicated by the 

 flatness of its cells. At nn is seen one of the large modified 

 nuclei, which has persisted unusually late. 



In Fig. 22 I give a section of the indusium seen in sur- 

 face view in Fig. 5. The spreading of the cells has pro- 

 gressed till the organ lies like a saddle on the ventral face of 

 the tg^^, covering nearly half of its circumference. The serosal 

 layer (s) is, of course, unaffected. The outer indusium {am'^) 

 is stretched to such an extent that its cells are united only by 

 an exceedingly thin and in many places, almost imperceptible 

 layer of protoplasm. The inner indusium now consists of a 

 single row of cells, instead of two rows as in the preceding 

 stage. It is in about the same state of tension as the outer 

 layer in Fig. 19. 



3. TJie Development of the Embryo from the Time of its 

 Reaching the Dorsal Yolk to Revolution. 



In the foregoing paragraphs the development of the embryo 

 was traced to Stage E, when the germ-band hangs festoon-like 



