No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 8 1 



the envelopes subserve a distinct function, but as soon as the 

 germ-band has invested the yolk with its own ectoderm, they 

 have become functionless, or rudimental. Long before this 

 time, in fact ever since their completion, the envelopes show 

 no traces of cell-division. Moreover, their involution into the 

 yolk or complete shedding shows conclusively that their mor- 

 phological value is at this time reduced to ;///. Whether both 

 envelopes are shed instead of being drawn into the yolk, or 

 whether one is shed and the other drawn into the yolk, may 

 depend to some extent on the ease with which the pleural 

 folds can close without their temporary assistance. But which 

 of these processes shall occur in a given insect is probably a 

 matter of no vital importance to the embryo, and has prob- 

 ably played no role in the struggle for existence. The involu- 

 tion of the envelopes, it is true, may add assimilable matter to 

 the embryo, but enough energy to counterbalance this addition 

 is probably consumed in metabolizing the dead cells. Hence 

 the adoption of this process may be of no greater advantage to 

 the embryo than the complete sloughing of the useless en- 

 velopes. 



The insect envelopes, therefore, present only another case of 

 an organ which has become specialized for a particular function 

 at the expense of its formative power. This same phenomenon 

 recurs in insect ontogeny. During cleavage certain cells are 

 segregated for the express function of yolk-metabolization (vi- 

 tellophags), while the remaining cells go to form the blastoderm. 

 Later the cells of the blastoderm separate into those of the 

 germ-band proper and those of the specialized envelopes. Still 

 later, if the insect be metabolic, another splitting occurs, a por- 

 tion of the hypodermis being set aside in the form of the imag- 

 inal disks to supplant the specialized primitive larval hypoder- 

 mis. The formative material of the insect, like that of other 

 organisms, thus undergoes a successive splitting into a special- 

 ized and a comparatively non-specialized portion. The former, 

 being incapable of metamorphosis, is cast off or broken down, 

 while the latter persists until a new segregation takes place. 

 The analogy of this process to that occurring in rhizomatous 

 plants, Polyzoa, etc., need not be pointed out in detail. 



