22 
W. F. I’UHCELL. 
Formation of the spiracle. — After the appendage has attained 
its greatest elevation (genei’ally late in the 3-furrow stage) the 
whole region between the three oldest furrows begins to sink 
below the level of the appendicular posterior surface by a for- 
ward moveineiitj causing* it to be over-topped by the distal edge 
of the appendage (fig. 16a). Tliis sinking movement^ which 
must not be confounded with the formation of the pulmonary 
folds described further oip commences next to the pulmonary 
sac, and the latter thus comes to include first the third furroAv 
(fig. 16b), then the second, and finally the first, while the 
common opening becomes the spiracle fig- 13^ which com- 
pare with figs. 13a and 13b of the same series). 
Meanwhile that portion of the body epithelium which lies 
immediately in front of each of the four appendages in a row 
becomes absorbed into the anterior side of the appendage 
(compare figs. 4, 5, and 6), so that the four appendages appear 
closer together, while the original opening of the pulmonary 
sac comes to lie at the bottom of the groove so formed between 
the first and second appendages. 
The lateral (afterwards dorsal) end of the spiracle is the 
first to be formed, and is already clearly elefined immediately 
after the appearance of the pulmonary sac (fig. 14). The 
jirogressive development of the lateral part of the spiracle 
may be followed in figs. 8g, 12 , and 16c. In the latter embryo 
the surface posterior (fig. 16c) and dorsal (fig. 16n) to the 
lateral end of the spiracle is almost on a level with the distal 
surface of the first appendage. The medial (later ventral) 
region of the spiracle remains open and undefined for a much 
louger time. 
Sinking of the appendage. — This is a very simple process 
and begins about the 5-6-furrow stage {St. 5). The anterior 
and ventral sides become more slanting, so as to pass, like the 
dorsal side, more and more gradually over into the adjacent 
body surface, while the appendage itself decreases in eleva- 
tion and sinks graduall}' into the body, until tinally only a 
slight convexit}' in front of the spiracle marks its former 
])osition. (Compare fig. 13b, with five pulmonary furrows. 
