The First Abdominal Segment of Embryo Insects. 
If 3 
PART THIRD. 
The question as to the original function of the pleuropodia must needs 
have suggested itself to all investigators who have met with these conspic¬ 
uous organs in insect embryos. Naturally enough, each investigator has 
sought an answer in the particular insect studied, in most cases never sus¬ 
pecting that so simple an organ as a pleuropodium could have undergone 
much modification and have assumed in forms unknown to him a structure 
calculated to render his narrowly based theoretical conclusions untenable. 
In view of the ectodermic origin of the pleuropodia, they may be said to 
have had one of three functions: they were either respiratory organs, sense 
organs or glands. Hence, owing to the fact that limited observation has 
precluded any general survey of the pleuropodia throughout the whole 
Hexapod group, different investigators have advocated one or the other of 
these functions, each being guided to his particular view by the special in¬ 
sect to which he devoted his attention. Thus Graber has become an advo¬ 
cate of the gill hypothesis from his observations on Melolontha , a form in 
which the pleuropodia are in many ways singularly specialized; Cholodkov - 
sky, perhaps impressed by what he supposed to be a facetted surface on the 
pleuropodia of Blatta — in reality a phenomenon due to his use of reagents 
— believes the pleuropodia to be sense organs; while Nusbaum has been 
most naturally led to regard the modified appendages as glandular organs 
by his observations on Meloe. 
1 shall proceed to a consideration of the three theories advocated up to 
date, briefly examining into the reasons which have influenced their ad¬ 
herents, and finally settling on the gland theory as to me the most probable. 
With this last theory none of the observed facts are in contradiction—• 
while as much cannot be said of the gill and sense organ hypotheses. 
A. The Gill Hypothesis. 
Ratlike, the first to find pleuropodia, was also the first to assign to them 
a function [’44]. Their peripheral position, the delicacy of their surfaces, 
their close adherence to the egg-membrane, which he thought due perhaps 
to some sticky substance, and the further fact that they contained cavities 
filled with what was very probably blood, made Rathke believe that he was 
dealing with respiratory organs. He supposed, moreover, that these organs 
functioned during embryonic life. The embryos of Gryllotalpa, he says, 
require a great deal of air, on which account they are deposited in spacious 
subterranean chambers. When simply buried in the earth, the eggs decay. 
Ayers followed Rathke in his interpretation of the problematic append¬ 
ages. [’84], Of late Graber [’88] though dealing with these organs at 
8—A. & L. 
