The First Abdominal Segment of Embryo Insects. 117 
4. In some insects at least [ Blatta, Periplcineta, Xiphidium, Stenoboth- 
rus\ the chitinous cuticle does not cover the pleuropodia, even after invest¬ 
ing the body of the embryo. On the supposition that we are dealing with duct¬ 
less glandular organs, the reason for this is obvious. The secretions of cutan¬ 
eous glands cannot penetrate a thick and unmodified layer of cliitin, so we 
find gland cells covered with a cuticle the [chemical ?] structure of which 
departs from that of ordinary chitin „ This cuticle is also thinner than that 
secreted by the unmodified liypodermis cells ol the general integument. 
A good example of such attenuation and modification in the molecular 
structure of the cuticle covering ductless cutaneous glands is furnished by 
the collophore of Anurida maritima described below. Now the first cuti¬ 
cle shed by the embryo in the egg must be regarded as the attenuated rudi¬ 
ment of what was formerly a much thicker cuticle shed by the insect in 
some post embryonic stage of existence. Supposing that the cuticle origin¬ 
ally covered the pleuropodia of such an ancient insect had been more 
delicate than that covering the remainder of the body, it would cease to be 
secreted in the embryos of existing insects because reduced to such exces¬ 
sive tenuity. There is still another possibility which might account for our 
not finding a chitinous cuticle on the pleurododia: the secretion of these 
organs may itself be some chemical modification of the chitin, which cov¬ 
ered the appendages before the peculiar differentiation of their cells set in. 
5. The lack of any apparent innervation to the pleuropodia, though 
adducible as a fact against the sensory nature of the organs, is just what 
we should expect on the supposition that they are glandular. The difficul¬ 
ties encountered by histologists in tracing the innervation of glands is well 
known. 
6. The manner in which some pleuropodia degenerate suggests what is 
known to take place in many glands that indicate their relation to epithel¬ 
ial structures by secreting their own broken-down cells. The milk glands 
of the mammalia and other cases will suggest themselves to the reader. 
7. The structure of the pleuropodia described up to the present, though 
considerably diversified, is in all cases consistent with a glandular func¬ 
tion. 
Having reached the conclusion that the pleuropodia functioned as glands 
in ancestral insects, I have probably made the utmost use of the few facts at 
my disposal. The word ‘ ‘ gland,” however, is so indefinite, since the special 
functions of glands are so numerous, that, stopping at this point, the hypo¬ 
thesis is still very vague. For the sake of giving it clearer outlines, I will 
elaborate still further, though aware of the dangers I incur in descending 
to particulars. 
What then was the special glandular function of the pleuropodia in primi¬ 
tive insects? In seeking an answer to this difficult question we are pursuing 
the most logical course when we muster the different classes of glands ob¬ 
served in air-breathing Arthropods, and select for special consideration the 
class that presents the greatest variation in structure and the widest dis- 
