12 WHEELER. [Vol. VIII. 



Returning to consider the indusium, we find that it begins 

 to increase in size before the embryo's head leaves the 

 ventral face. The organ stains much less deeply, and even in 

 surface view its expansion may be seen to be due to a flatten- 

 ing of its component cells. In Fig. 5 (Stage E) is represented 

 an embryo merged in the yolk up to the first maxillary seg- 

 ment. The indusium extends around on either side nearly 

 to the middle of the lateral face of the ^gg. Either the tran- 

 sition of the embryo takes place rapidly or the organ changes 

 very gradually, for the latter is in about the same stage after 

 the embryo has become established on the dorsal surface. 

 The manner in which the expansion of the indusium is 

 brought about will be apparent when I come to describe its 

 structure in sections. 



b. THE INDUSIUM IN SECTION. 



As will be seen from the preceding account, the indusium 

 is simply a circular thickening of the blastoderm, situated 

 in the median line, between and a little in front of the 

 procephalic lobes. It does not arise as a part of the ventral 

 plate but as a separate centre which is at first merely a 

 cluster of blastoderm cells that have changed from the pave- 

 ment to the cubical or columnar type. This centre is further 

 increased in breadth and thickness by caryokinesis. In the 

 earliest stages examined, sections of the organ show the same 

 cell-structure as sections of the procephalic lobes. 



Median longitudinal sections of the embryo in Stage C are 

 interesting as showing the relations of the indusium to the 

 embryo and its envelopes. I reproduce such a section in Fig. 

 21, PI. III. Here the organ (/. 0) appears as a large flattened 

 cell-aggregate somewhat thinner in the centre than nearer its 

 periphery. Owing to the shape of the mass, the median cells, 

 as indicated by their nuclei, are arranged with their long axes 

 perpendicular to the flat outer surface of the organ, while the 

 cells of the thickened lateral portions become gradually more 

 oblique till those on the extreme periphery assume the same 

 position as the serosa cells (j.). The nuclei are most frequently 



