402 THE DEVELOPMENT OF THE CHICK 
behind the external glomeruli of the pronephros. The lips of 
this groove then approach and fuse on the fifth day, so as to form 
a tube which soon separates from the ridge. This process, how- 
ever, takes place in such a way as to leave the anterior end of 
the tube open and this constitutes the ccelomic aperture of the 
oviduct, or ostium tube abdominale. Moreover, the closure of 
the groove does not take place uniformly, and one or two open- 
ings into the Millerian duct usually occur near the ostium on 
the fifth day. Typically, however, these soon close up, though 
persistence of one of them may lead, as a rather rare abnormality, 
to the occurrence of two ostia in the adult. There is no ground 
for the view (see Balfour and Sedgwick) that the two or three 
openings into the anterior end of the Miillerian duct correspond 
to nephrostomes of the pronephros; they are situated too far 
posteriorly and laterally to bear such an interpretation. 
The anterior part of the Millerian duct is thus formed by 
folding from the epithelium of the tubal ridge; it constitutes a 
short epithelial tube situated between the Wolffian duct and the 
tubal ridge, ending blindly behind. The part thus formed is rela- 
tively short; the major portion is formed by elongation of the 
anterior part, which slowly grows backwards between the Wolffian 
duct and the tubal ridge, reaching the cloaca on the seventh day. 
The growing point is solid and appears to act like a wedge sepa- 
rating the Wolffian duct and the tubal ridge, being thus closely 
pressed against both, but apparently without receiving cells from 
either. Balfour’s view, that it grows by splitting off from the 
Wolffian duct or at the expense of cells contributed by the latter, 
has not been supported by subsequent investigators. A short 
distance in front of the growing point the Miillerian duct receives 
a lumen, and mesenchyme presses in from above and_ below, 
and forms a tunic of concentrically arranged cells around it 
(Fig. 221). 
The Miillerian duet thus begins to project above the surface 
of the Wolffian body, and, as it does so, the thickened epithelium 
of the tubal ridge becomes flat and similar to the adjacent peri- 
toneum; whether it is used up in the formation of the mesen- 
chymatous tunie of the epithelial Millerian duct is not known. 
Up to this time the development is similar in both sexes and on 
both sides of the body. 
In the male development of these ducts ceases on the eighth 
