3 
492 = The American Naturalist. [May, 
by a long muscle, the retractor penis, which runs from the point of the 
penis through its interior back for a considerable distance in the tail ; 
there it is attached to one of the caudal vertebra. The action of this 
muscle is such as to turn the penis inside out,—as the finger of a 
glove could be turned,—back into its place under the skin. At times 
of copulation the organs are everted, chiefly by an influx of blood into 
their erectile tissue. 
The copulatory organs first appear in embryos of about the sixth 
week. At this stage most of the important organs of the body are 
formed ; the body has completely closed in, except at the umbilicus, 
the food-yolk not being entirely consumed until a much later period. 
The wolffian ducts open into the cloaca, but the ureters have not yet 
grown so far back. . 
The first appearance of the penes takes the form of two ridges, 
one on either side of the body, and extending from a point a little 
ead of the cloaca to about opposite its posterior end. In sections 
across the body at this stage (Fig. 1, ø) these ridges can be seen as 
bulgings of the body-wall. These bulgings are filled (Fig. 2) with an 
undifferentiated mass of mesoderm cells, similar to and continuous 
with those composing the body proper. The whole is covered wit 
the characteristic double-layered epithelium. . 
The further growth of the penes from these ridges reminds oe 
strongly of the manner of formation of the posterior appendages hat 
the chick, from the end of the wolffian ridge. The extreme posterior 
end of each ridge swells out, forming a rounded prominence, oes 
“‘nipple-shaped swellings” of Rathke. The remaining pean fs 
each ridge grow no further, and finally disappear. In embryos © 
week later no trace of them is to be found. 
