156 
Heligmosomum muris Yokogawa 
become cuboidal in shape and are transparent (Figs. 12 and 13, ed, Plate IX). 
this part is found always along the ventral side of the intestine and its pos¬ 
terior end projects into the cloaca at the ventral side of the rectum. 
(2) The External Sex-Organs. 
General discussion. The term external sex-organs” is used to refer to 
the organs connected with the reproductive processes in the male which do 
not develop from the genital primordium. 
At the beginning of the parasitic life the number of cells in the tail and 
around the rectum of the male increases. As development prqceeds these cells 
continue to increase in size and in number, and become re-arranged in a definite 
manner. It is from definite groups of these cells that the spicules, gubernacula 
and the rays of the bursa develop. 
The development of the hursa. The cell group found in the posterior end 
of the male seems to have originated from the lateral bands and the muscle- 
cells, since they are located on the sides and show no relation to the ventral 
and the dorsal walls of the body as early as 24 hours after infection. With 
development these cells increase in size and number and a space is formed 
around the rectum (Fig. 9, rs, Plate IX). In the beginning of the first moulting 
the cells of the subcuticular layer at the ventral side of the tail region divide 
and join the other cells of this region, so that toward the end of the first para¬ 
sitic stage the posterior end of the male filled up with clear cells with round 
nuclei (Fig. 10, Plate IX). During the moult following this stage these cells 
show a definite arrangement as shown in the figure (Fig. 11, Plate IX). Early 
the cell group which is situated just behind the rectum becomes separated and 
differentiated into elongate conical or spindle-shaped cells (Fig. 11, cs). The 
length of these cells increases as development proceeds and later they produce 
the spicules (Figs. 11-14, cs, and 15, s, Plate IX), while the other cells of this 
region later form the bursal rays. 
After the completion of the second moult the space which has been formed 
around the rectum gradually expands from its posterior end, particularly 
toward the ventral and the lateral sides (Figs. 12-15 and 19, Plates IX and X). 
As this space expands the posterior end of the male becomes inflated into a sac 
lined with cells. Toward the end of the second parasitic stage the cells which 
line the posterior part of this bursal sac produce small finger-like projections 
which are the beginnings of the rays of the bursa (Fig. 16). At this time each 
ray consists only of the projection from a single cell. These projections already 
represent the number and have assumed the position of the fully developed 
rays. The bursa therefore consists only of the inflated posterior end of the male 
which is lined by cells and is separated from the true posterior end of the worm 
by a considerable space (Fig. 19). The rectum (Fig. 19, r) still opens at the 
posterior end of the bursal sac. During preparation for the third moult, the 
lobes of the bursa become separated into their final form and the closed bursal 
sac now consists only of the old cuticula. After the shedding of this skin the 
