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At the posterior pole of the egg is a very characteristic structure (Plate 
LXVI, 34), to which Graber (1872:165) gave the name ‘“ Histigma.’”’ The 
earliest description of this structure is that of Leuckart (1855:139, 141), 
who observed it on the eggs of Pediculus capitis and Haematopinus 
suis, and it was seen also by Landois (1864:15) on the egg of Phthirius. 
Gross (1906:372) has given the first detailed description of it and figured 
its structure. The egg stigma forms a roundish swelling on the chorion 
and is pierced by numerous thin-walled canals, which narrow toward 
their inner ends and converge, to one side. Gross studied its formation 
in detail, and in young stages found the egg follicle closed by a plug 
extending far into the interior of the yolk, but as growth proceeds the 
plug becomes leveled. The nuclei are small and the inner ends of the 
cells are drawn to a point. These inner ends are cut off from the cells 
in a characteristic manner and the nucleus is drawn to the outer wall, 
while between them is a zone of protoplasm in which cell boundaries 
can no longer be recognized. Between the detached inner pieces begins 
the deposition of chitinous substance, and this appears as fine striae, 
while at the exterior, where the deposition has become more advanced, 
the thin, chitinous lamellae have lost their color. Then are formed in 
the region of the stigma the endochorion and the exochorion. The stigma 
is now completely developed. The point formed by the pointed ends of 
the cells still remains attached inside, cell walls can be recognized, and 
it can be seen that each cell forms a canal. 
No satisfactory ~biological interpretation has been found for the egg 
stigma, a structure found in no other insect order. Earlier authors 
advanced three views, none of which has proved satisfactory. Leuckart 
(1855:139) and Melnikow (1869:154) regarded it as an attachment disk; 
but if this were its function, why should it be pierced by canals in most 
cases? Kramer (1869:462) regarded it as the true micropyle; but in 
some species, for example Nirmus, the canals do not pass to the inner 
ends of the cells. Graber (1872:163) interpreted it as a means of aeration 
for the eggs and so named it; but in most cases it is covered over by 
secretion. The closing of the pores by secretion would be essential in 
Pediculus, since, according to Sikora (1915:530) and Nuttall (1917 d:148), 
the embryo escapes from the egg by pumping air through its alimentary 
canal in order to increase the pressure in the egg and force open the 
operculum. 
