THOMPSON: ANATOMY OF MOSQUITO. 
191 
cells are swept away while van Rees (’88) believes that a consider¬ 
able part of the older wall remains. In Culex the appearance of the 
regenerative changes in the neighborhood of the Malpighian tubules 
before the remainder of the gut is involved, and the formation of 
an anal ring of new cells — whatever their ultimate source — fur¬ 
nish interesting parallels to the conditions found in Musca and Gas- 
trophilus. The resemblances of course mean no more than the 
presentation in varied degree of certain tendencies which exist 
among the Diptera. A closer genetic connection is not implied. 
As an illustration of the improbability of such a connection we 
may take the changes which occur in the colon of Culex. The 
development of this region recalls the reconstruction of the hind 
gut in the more specialized Hies from anlagen at either end of the 
canal. But the processes are not really comparable with those 
found in Musca or Gastrophilus and when we consider the simple 
metamorphosis of the remaining hind gut there is marked and 
peculiar specialization. The colon of Culex is destroyed and 
replaced by a tube formed from a backward growth of the cells 
of the ileum and a forward growth of the cells of the rectum. 
The proliferation begins as soon as the iliac and rectal epithelia 
are reconstructed, i. e., about the seventeenth hour. At this time 
the cells of the shrunken colon are spongy, and have separated 
from one another. Their nuclei have diminished in size but are 
intact (pi. 17, fig. 52, co). The invading cells enter the lumen (pi. 
17, fig. 59), and active mitosis is observable at and near the head of 
either advance. The two cell armies seem to progress with equal 
speed. As soon as the advance is well under way, the nuclei of the 
colon cells lose their reticular structure and reduce to masses of 
chromatic material within the nuclear membranes. Nuclei and 
cells, however, remain distinct until individually displaced by the 
invading cells, and then quickly liistolyze. In a nineteen-hour 
pupa, cells already displaced lay outside of the new epithelium as 
a mass of fragments, while cells still in position remained distinct 
units. By the twentieth hour of pupal life (pi. 17, fig. 53) the 
new canal \ilc) is completed and the old colon (co) is represented 
by debris. This is quickly absorbed without the intervention of 
phagocytes. The muscles of the larval colon liistolyze in situ. 
The muscles of the new ileo-colon are developed from myoblasts, 
probably from the ileum and rectum. 
