1891.) Zoology. 493 
These swellings, however, continue to enlarge, and form the penis 
proper. They grow very rapidly in length, and attain their full size 
in about three weeks. They are then equal in length to three-quarters 
the vertical diameter of the body. 
Simultaneously with the growth of these swellings a differentiation 
of the mesoderm cells in their interior begins. The ce 
exterior gradually lose their round form, become more and more 
angular, their ends prolonged more and more into slender processes, 
Fig. 1r. 
until, in my latest sections of the black snake, they can be seen to 
be well on their way toward the formation of connective tissue, 
Among these cells clear spaces with a regular outline appear (Fig. 4,7). 
These are lined with a layer of flat cells, similar to those Neumann 
describes as lining the blood cavities in the adult penis. They are 
the beginnings of the blood spaces, which, with their connective tissue 
walls, compose the erectile tissue in the adult. 
While this change into connective and erectile tissue is going on 
among the outer mesoderm cells, those on the interior undergo a 
differentation which leads to the formation of the refractor penis. In 
embryos of about the seventh week the first rudiments of this muscle 
may be seen as a thickening or crowding together of the mesoderm 
cells in the middle of each organ (Fig. 3, 7). These cells elongate, 
become nearly elliptical in form, and arrange themselves in rows, thus 
giving rise to the muscle fibres. 
In the adult penis there are two sets of these fibres, a large, inner, 
longitudinal band, with an incomplete circular band surrounding it. 
Am. Nat.—May.—6. 
