2 6 WHEELER. [Vol. VIII. 



observable in a large number of eggs. The variations may be 

 tabulated thus : — 



1. Variations in size. Normally the indusium is of the 

 same size as one of the procephalic lobes (.2 mm. in diameter) 

 so that the head of the embryo resembles a clover leaf as long 

 as the organ is attached to it. When the chorion is removed 

 the organ may be distinctly seen with the unaided eye as a 

 milk-white spot on the translucent yolk. Occasionally, how- 

 ever, embryos will be found in which it is less than .1 mm. in 

 diameter, and all variations between this and the normal size 

 may be observed. 



2. Variations from the typical circular form. These varia- 

 tions are very numerous and may be regarded as belonging to 

 two classes. In one class the indusium is rounded in outline, 

 while in the other it is ragged and more or less irregular. 

 To the first class may be assigned the oval, cordate and 

 multilobulate varieties not infrequently observed ; to the second 

 belong a number of irregularly stellate and rhizopod-like forms. 

 In one of my preparations, midway between the two classes, 

 the indusium is evenly rounded anteriorly and ragged poste- 

 riorly along that portion of its periphery which has just broken 

 away from the head of the embryo. 



3. There is a variation in the time at which the organ 

 is set free from the head. This cannot be proved directly 

 by observation of the organ itself, for it usually does not 

 begin to form the circular fold till after its isolation, but 

 differences in the embryo, especially in the prominence of the 

 segments and appendages, show that the organ remains at- 

 tached to the head in some cases longer than in others. 



4. Variations in the development of the circular fold. These 

 variations, alluded to above, are characterized by a greater or 

 less distinctness in the folds that give rise to the inner and 

 outer layers. All shades in the process may be found between 

 the distinct and comparatively rare method described and 

 figured (Fig. 3), and the more frequent and obscurer method 

 whereby the three layers are formed by a shifting of the 

 individual cells. 



5. Variations in number. I have twice observed two indusia 



