HISTOLOGY OF THE LAND-PLAN ASIANS OF CEILON. 
139 
entiated that it exhibited a well-formed cellular lining, just like that in Bipalium. 
The testes in Bipalium (Plate XII. fig. 1) are arranged in pairs along aline just external 
to that occupied by the oviduct. They commence a short distance behind the ovaries, 
and extend for about half the interval between the ovaries and the oral aperture. There 
are about twenty-four or twenty-five pairs ; they are separated from one another by short 
intervals, and communicate by means of lateral apertures with the vas deferens, which 
passes straight down the body along their inner side to reach the region of the penis. 
In transverse vertical sections of the body the testes are seen to occupy a position imme 
diately external to the main primitive vascular trunks in both Bipalium and Bliyncho- 
demns, and the vas deferens to lie immediately between the vascular trunks and the 
testis (Plate X. fig. 5, V. D.). The separate testes are in Bipalimn irregularly spherical ; 
they consist (Plate XIII. fig. 4) of a sac or wall and contents. The sac is formed, as in 
the ovary, of a fine but dense inner membrane, and an outer loose investment full of 
irregularly oval spaces, and probably in communication with the vascular system. The 
whole testis lies imbedded in a cavity amongst the longitudinal muscular fibres, as does 
the ovary. On the inner side of the spherical testicular sac is an opening, by means of 
which it communicates with the cavity of the vas deferens ; and it would appear that the 
basement membrane of the vas deferens becomes continuous with the inner membrane 
of the sac. There is a bulging out of the wall of the vas deferens opposite the spot 
where the testicular cavity communicates with it, and a sort of sinus is thus formed in 
it at this place. The vas deferens (Plate XIII. fig. 7) consists of a basement membrane 
lined with nearly rectangular nucleated cells ; these cells are much smaller than those 
of the oviduct ; and the vas deferens is oval in section, not circular, and has a wide lumen. 
Opposite the opening into the testis the epithelial lining of the vas deferens is thickened 
in a remarkable manner. The epithelium is not ciliated, as is that of the oviduct. The 
cavity of the testis is always divided into two regions — an outer broad zone full of imma- 
ture cells, and a central cavity in which are ripe parent cells of spermatozoa with sper- 
matozoa attached to them in process of development and also in the free condition. 
The outer zone is so well defined from the inner space that it would almost appear as if 
a thin membrane were reflected back from the point of union of the vas deferens with 
the testicular sac-wall, and separated the two regions. I could not, however, make sure 
of the existence of such a membrane. It appears that the cells, as they become more 
and more developed within the outer zone, pass gradually outwards away from the 
opening into the duct till they reach a point opposite this opening, by which time they 
have reached a high state of maturity ; hence they pass into the inner space of 
the testis, and here give birth to the spermatozoa. It appeared, from the study of a 
large number of sections, that the large parent cells of the spermatozoa were formed by 
the aggregation of a number of smaller rounded cells ; but such a point could not, of 
course, be determined with certainty from preserved specimens. The spermatozoa are 
formed at the periphery of the large parent cells as in Annelids, and at first are provided 
with pear-shaped heads, which, however, are absent in mature spermatozoa, since some 
of them were present in the spermatozoa crowding the vas deferens. 
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