172 EDWARD A. BOYDEN 
The final stage in the formation of the fenestra, ending the 
period of disintegration, is shown in figures 14 and 15, of an 
8-mm. embryo of forty-eight somites (3 days and 6 hours). The 
entire roof of the cloaca, between the wolffian ducts and the 
anal side of the caudal intestine, has been denuded of epithelium, 
leaving a considerable gap bounded only by mesenchyma (dash 
line, fig. 14). The connection of the cloaca with the caudal 
intestine has been lost, and the latter, together with the primi- 
tive-streak mass, is now rapidly disintegrating at the ruptured 
ends. As a rule, degeneration does not spread any farther ceph- 
alad than recorded in figure 5. But occasionally it extends 
much farther, and is probably instrumental in producing irreg- 
ularities in the dorsal wall, which will be discussed later, in the 
section dealing with accessory diverticula. 
The cytological changes involved in the formation of the fenes- 
tra include the necrosis of the epithelial cells, their removal by 
phagocytes, and the reaction of the surrounding mesenchyma to 
the denuded area. As seen in ordinary serial sections, the first 
step in the disintegration of the flanks of the cloaca is aslight 
-oedema of the epithelium which causes the cells to spread apart. 
As these become necrotic, the cytoplasm becomes finely granular 
and then vesicular and the nuclei pyenotic. At this stage the 
epithelium presents a confused histological picture due to the 
simultaneous degeneration of so many cells. But almost imme- 
diately the cells in regions y and 2g (fig. 2) are resorbed, leaving a 
gap in the wall covered only by mesenchyma. At first the 
mesenchymal cells appear to congregate about the region, as if 
to plug up the opening, and this continues as long as there is an 
abundance of necrotic tissue. During the stage when the wall 
is a fenestrated membrane the mesenchyma may even invade the 
cavity. This is especially true of the caudal intestine which is 
eventually replaced by mesenchyma which has grown in through 
rifts in the sides and filled the cavity before the walls have been 
completely removed. 
The most favorable time for appear the cytological changes 
is after the gap has been formed on each flank of the cloaca, but 
before the roof of the cloaca thus isolated has itself been removed. 
