1 4 WHEELER. [Vol. V 1 1 1 . 



oval, evenly rounded nuclei with one or two nucleoli and their 

 chromatin is distributed in what I take to be the typical resting 

 reticulum. The caryolymph, or Kernsaft, is faintly stainable. 

 On the other hand, in the nuclei of Fig. 24 the nuclear wall is 

 very irregular, the caryolymph much more limpid and refrac- 

 tive and the chromatic reticulum has coarser meshes. The 

 chromatic nodes of the reticulum are larger than in Fig. 23 

 and seem to be applied to the indentations of the nuclear wall. 

 Nucleoli appear to be absent. These specialized nuclei also vary 

 greatly in size. In a series of sections it is easy to find nuclei 

 intermediate between the two extremes here described, 

 being evenly rounded but with colorless caryolymph and 

 coarse chromatic reticulum. A cluster of four such nuclei 

 is shown at nn'^ Fig. 17. These intermediate forms, occurring 

 as they usually do, between the normal and the modified 

 nuclei may be taken to indicate that the nuclei of the extreme 

 types are genetically connected. Some of the normal nuclei 

 probably leave their respective cells in the median portions of 

 the organ and move up into the syncytial protoplasmic layer, 

 undergoing the modification in structure during their emigra- 

 tion. When they have reached their destination they are 

 perhaps broken down and converted into protoplasm. Certain 

 it is that later no traces of them are to be found in the 

 indusium. I do not believe that I am here considering collapsed 

 and distorted caryokinetic figures, as these delicate structures 

 are quite faithfully preserved in eggs killed by means of heat. 

 The distorted nuclei are not confined to the indusium but 

 occur also in the ectoderm of the embryo itself. 



When the organ has reached the state just described it 

 usually separates from the head of the embryo; it may, how- 

 ever, remain attached for some time longer. Like the embryo 

 it is now an isolated body lying on the yolk; but unlike the 

 embryo it is still only a part of the serosal envelope (which is 

 itself only the extra-embryonal portion of the blastoderm). The 

 serosa is a closed sack enveloping the whole yolk and the 

 indusium is simply a swelling at one point on its inner 

 face. (Fig. II, A.) The process of envelope formation which 

 now begins in the indusium is much less clear than the cor- 



