1943] 
Corpus Allatum of Sheep Ked 
3 
tension of the developing gut. The cells comprising the gland 
have distinct cell boundaries and show no indications of form- 
ing a syncytium. While there is some variability in nuclear size, 
there does not appear to be any differentiation into more than 
one cell type. Measurements of the diameters of 100 nuclei 
from both glands of a single larva, representing approximately 
one fifth of the total nuclei, give a unimodal distribution curve 
with a range of 8.5 to 13.0 microns and a mean of 11.0 microns. 
Nucleoli are conspicuous and the chromatin evenly distributed 
following the fixatives employed. The cytoplasm in the cells at 
this stage exhibits conspicuous vacuoles which are generally 
peripherally located. There are usually one or two large vacu- 
oles and a number of smaller ones, which give the cytoplasm 
the appearance of a reticulum, especially after Bodian’s tech- 
nique. As compared with the young larva, the cells representing 
the corpus cardiacum have been greatly reduced. From cyto- 
logical evidence it would appear that their principal activity 
occurs during larval development. 
The Pupa. It would be desirable to have a closely spaced 
series of pupae, for during this time very active changes occur 
in the cells of the corpora allata. Unfortunately only a few 
pupae (3 “white” pupae, 1 of about two weeks of age and 2 nearly 
mature pupae) were available, but these all agree in showing a 
marked reduction in the size of the corpora allata. Not only does 
the cytoplasm become reduced, but nuclear size also decreases. 
This decrease amounts to about 25 per cent of the former vol- 
ume. Rhythmic increase in nuclear size is a not uncommon 
phenomenon in insect tissues (see Geitler, 1940, for review), 
but a striking decrease as found here is unusual. Moreover, 
there is a decrease in number of cells, a change reminiscent of 
that undergone by the mammalian thymus during development, 
and, as in the thymus, the mechanism of the reduction of the 
cell number has not been determined. 
Pupal development of Melophagus occupies approximately 
three weeks. The puparium is white in color immediately after 
formation, but it hardens and darkens within about four hours. 
The corpus allatum of the white pupa is not unlike that of the 
fully grown larva. Considerable vacuolization of the cytoplasm 
is evident; the cells are approximately 15 microns in diameter 
and the nuclei average about 8.7 microns. As development pro- 
ceeds, the cells lose more and more cytoplasm until, by the end 
