102 
Wisconsin Academy of Sciences, Arts and Letters. 
are generally circular in section and pursue irregular courses throughout 
the cell substance, while the spaces are developed by the separation of ad¬ 
jacent cell walls and are irregular in outline and occur at varying distances 
from each other. The gill pad is essentially a single layered sac, with a 
much constricted neck, evaginated from the pleural region of the abdomen. 
The protruding organ is flattened against the body of the embryo and by 
this means the cells are rendered spindle-shaped. The nucleus of each cell 
lies in that part of its cell which is farthest from the constriction of the 
organ. The cell wall gradually tapers to a point and ends near the neck. 
The cells are bent in various ways depending upon the relations of their 
nuclei to the wall of the pad.” 
Ayers seems not to have recognized the identity of the abdominal ap¬ 
pendages and what he calls the “ gill-pads.” On plate 18 he figures [Figs. 
8, 19, 20, 23, ab. p.] the first pair of abdominal appendages as digitiform 
and arising in a line with the thoracic and remaining abdominal appenda¬ 
ges. In Fig. 20 these organs are seen with tips directed outward, while 
the ends of the thoracic appendages converge. Concerning the abdominal 
appendages he writes: 
“ Soon after the mesoderm has extended into the hollow appendages, 
there appear successively a varying number of abdominal protuberances 
exactly similar to the maxillary and thoracic appendages in their earliest 
stage of growth. Of these only two pairs ever reach any considerable de¬ 
gree of development, they are the first and the last abdominal. The for¬ 
mer grows to the length of the mature mandibles and then atrophies. It 
varies in shape from a finger-like process [pi. 18, fig. 17] to a lobed out¬ 
growth, and in the later stages is covered by the last thoracic appendage.” 
The last sentences imply that the organ undergoes dissolution in situ and 
this is further sustained by Fig. 22, where what would seem -to be the ir¬ 
regular remains of the appendages are seen through the translucent rneta- 
thoracic legs. Evidently Ayers lost sight of the appendages after they 
had been covered by the last thoracic legs and when they passed out from 
under these and made their appearance as enlarged and peculiarly modified 
organs high up on the pleural wall, they were regarded as organs having 
no relation to the abdominal appendages. Ayers no longer letters the ap¬ 
pendages withab. p., but with a new reference, A, and, what is more con¬ 
clusive, says that the pleuropodium “ is essentially a single-lavered sao 
with a much constricted neck, evaginated from the pleural region of the 
abdomen.” 
The section of the “ gill” described and figured by Ayers [Figs. 13 and 
14, pi. 22] “before its atrophy [or absorption]” shows the cells after the 
setting in of degeneration. The figures are in every way comparable 
to the figure which I have given of Periplaneta (see above, page 92, also 
fig. 19, Plate II). Fig. 14 shows the dorsal wall of the embryo covered 
over and the heart completed, a stage in which the pleuropodia have 
passed beyond their maximum development which is attained just before 
