AVIAN CESTODES. 881 
observed the formation out of the medullary parenchyma of a 
definite structure that can be called a separate organ, since the 
modified region of the proglottid extends over the whole medullary 
parenchyma and includes the dorsal vascular tube. 
J have also figured in my paper referred to stages which are 
anterior to that which has just been redescribed. In text-fig. 26 
of the paper referred to*, three proglottids somewhat younger 
are represented in sagittal section. A glance at this figure 
might convey the suggestion that a definite paruterine organ of 
limited extent lay in each of these proglottids, narrower at one 
end (where the letter “'l’” in the diagram is placed) and wider at 
the other. Furthermore, the slightly twisted outline of the 
(alleged) paruterine organ recalls that of, for example, Rhabdo- 
metra nullicollis t. A more careful scrutiny of these sections, 
however, brings to light the following faets which are of importance 
in the matter. Although, as depicted in my illustration, the edge 
of the (alleged) paruterine organ is apt to be wavy and thus to 
create inequalities in its diameter, suggestive of a solid body of 
irregularly curved outline, it will be found that the waviness is 
closely followed by the layer of transverse musculature which 
separates, in this as in other tapeworms, the cortical and medullary 
layers. Unequal contraction during preservation is, as I think, 
responsible for this undulating disposition of the line of transverse 
muscular fibres. The object, however, of my figures referred to 
was not to show the structure of the medullary parenchyma but 
to indicate the position and relations of the uterus. ‘The minute 
structure of the medullary region in this stage is less modified 
than that of the older proglottids already referred to. The 
medullary groundwork is traversed by numerous rather stout 
muscular fibres, running mainly if not entirely in a dorso-ventral 
direction. These are very frequent, but are single and not 
ageregated into bundles excepting at the anterior end of the 
proglottid; here the testes of this segment in front are separated 
by a thicker layer of these muscles from the parenchyma of the 
ensuing segment, the groundwork is comparatively dense, and 
there are abundant nuclei. I have recognised that in those pro- 
glottids, as well as in the more mature ones, the dorsal water- 
vessels are included in the medullary tissue. In comparing this 
stage with the older one that has just been described, it appears 
that the latter differs only in the degeneration of the mus- 
cular fibres of the ground-tissue, which produces the more 
fibrous and, at the same time, laxer appearance of the medullary 
parenchyma, which, however, may be more resistant, and which 
is still further exaggerated in the distended perfectly ripe and 
detachable proglottids at the end of the worm’s body. This laxity 
favours the movement into the interior of the embryos from 
the uterus, which I have described in my paper as occurring in 
* P. 204. 
+ Ransom, Bull. U.S. Nat. Mus. No. 69, p. 29, fig. -21, 1909. 
