432 
F. A. POTTS. 
lu the ectoderm the nerve-cord is now a double structure, 
and opposed to it is a crowded mass of nuclei mainly 
developed in the basal ends of the ectodermal cell impinging 
on it. 
The stolons of this stage consist of a number of segments 
(fifteen or sixteen), but new segments continue to be added, 
both proximally and distally. A definite head has not been 
formed, although the tail segment is early established, and 
so there is as yet no sign of eyes or of a brain. 
In PI. 23 bis, fig. 16, we have a transverse section through a 
stolon of T. crosslandi (2), the tissues of which have 
attained to more marked development than any of the stolons 
of T. gemmipara. This has accompanied the lipening of 
the generative cells. The spermatocytes have escaped from 
the gonads, and lie freely in the body-cavity. The growth of 
these has accelerated the shrinkage of the fibrous core of the 
mesoblast, which now appears as a triangular split between 
the two layers of peritoneal epithelium. The other great 
diifei’ence has been brought about by the development of the 
parapodia and muscular system. The notopodia, with bundles 
of swimming setas, have appeared. The neuropodia project 
outward considerably, and their acicula penetrate the cavity 
of the segment almost to the centi’e. The four main longi- 
tudinal bands of muscle-fibres remain, but there are in 
addition, in each segment, dorsal transverse bands which 
supply the swimming setae, and ventral transverse bands 
which work the neuropodial setae, beside the muscles which 
move the acicula. A close examination of these secondary 
muscle growths shows their derivation of the four original 
muscle bands. 
The nerve-cord now shows a regular cellular investment, 
and so while the fibrous core is derived by ingrowth from the 
stock, the investing layer, which probably forms the per- 
manent nervous system of the stolon, is formed from the ecto- 
deimal epithelium. 
There occurs also, for the first time, a pair of conspicuous 
tubular glands in each segment, consisting of a single layer 
