OVARY AND OVARIAN EGG OF ANOPHELES 435 
IV. Development of the Outer Wall. 
The first portion of the outer wall to appear is that which 
forms the floats. This is secreted between two layers of 
epithelial cells which come to lie one above the other by a very 
specialized form of folding of the epithelium. 
During the earlier stages of the growth of the follicle the 
epithelium is of a typically cubical form, but later the cell 
divisions in two lateral areas become oblique, the obliquity 
being more marked towards the centre of each area. This 
process continues with further growth of the follicle (fig. 34) 
till one much elongated cell lies over the top of several (fig. 35). 
The underlying cells, however, do not lose their connexion with 
the tunica propria, but remain attached to it immediately in 
front of the end of the overlying cell. Finally, it is found that 
in the two lateral areas there are groups of very much elongated 
cells which lie almost parallel to the tunica propria. The float 
is secreted between the outermost of these and the one lying 
immediately under it (fig. 35). Hach corrugation of the float 
is produced by the secretion of the chorion over the outer surface 
of one of the much elongated underlying cells. 
It will be seen that this overlapping arrangement of the 
follicle cells is practically a fold of the epithelium. It is not 
an ordinary epithelial fold, however, as the basement mem- 
brane, i.e. the tunica propria, is not disturbed and does not 
take any part in the folding. 
The remainder of the wall makes its appearance shortly 
after the commencement of the formation of the floats. It is 
first seen as a simple and very thin membrane lying immediately 
under the follicular epithelium. Soon lccal thickenings are 
found on this membrane (fig. 35). These are the commence- 
ment of the processes. The thickenings become larger and 
grow into the cytoplasm of the epithelial cells. Numbers of 
such thickenings are formed under each epithelial cell, and 
the shape of the processes cannot therefore be determined by 
the form of the secreting cells in the manner which frequently 
occurs, e. g. in the corrugations of the floats. 
NO. 259 Gg 
