314 
MR. WILSON ON THE STRUCTURE AND DEVELOPMENT OF 
the latter bodies (transition ova) varied between tbW aa d tooo °f an inch in the 
short diameter, and y^o and TW9 in the long axis. The perfect ova are oval in shape, 
somewhat larger at one end than at the other, about twice the lengt h of their breadth, 
semitransparent, amber coloured, and composed of nucleated granules or cells inclosed 
in a thin and yielding membrane. They vary in length from to and in 
breadth from yyy to yyy of an inch. 
The first developmental change observable in the ovum is the disintegration of the 
tesselated plane of nucleated cells which constitute the surface of the ovum, and lie 
in contact with its investing membrane. These cells, which are at first polygonal in 
form, become rounded and larger in size, and the cells of the centre of the ovum 
acquire a considerable preponderance in bulk over those of the periphery. Subse- 
quently to these changes the cells assume a special arrangement, they become aggre- 
gated into a small oblong mass, which is curved at each end, and is not unlike the 
embryo of higher animals, while a space is formed between the trunk of the little 
mass and the membrane of the egg. The next stage would seem to be the rupture 
of the egg-membrane and the expulsion of the embryo, but this process I have not 
observed in operation, although I have seen on the one hand embryos which ap- 
peared to be just released from their investing membrane, while on the other I have 
detected egg-membranes evidently cast off by ecdysis. In some few of the ova I 
have observed adipose cells, some large and some small, mingled with the germinal 
cells. 
It is a curious fact, that in the development of the Entozoon foUiculorum. a re- 
markable embryonic form is met with which is peculiar to the long variety of the 
animal, and is not met with in the shorter kind. This embryonic body is elongated, 
rounded and bulky at one extremity ; smaller, somewhat tapering and blunt-pointed 
at the other, and enlarged towards the middle by the development from one of its 
sides of two oval-shaped prominences which subside gradually into the larger end 
and join the smaller end, either at a right or at a receding angle. This peculiarity of 
form enables us to divide the embryo into a body, which comprises the larger end 
and these two oval prominences, and a tail, which is the smaller and shorter end ; 
the principal changes in its form taking place, in the course of development, being 
the elongation of the tail and the enlargement or subsidence of the oval prominences. 
The dimensions of the embryo are syy to of an inch in the long, and to ygy 
in the short diameter. In structure the embryo presents a close analogy with the 
ovum, being composed of nucleated cells, having the same disposition and appear- 
ance, and enclosing in the centre of their mass a number of larger cells. The larger 
cells correspond in position with the broadest part of the embryo, and extend more 
or less in proportion to its development into the cephalic and caudal portions. They 
also vary in their size, the central cells being the largest, while those which surround 
them are small and more numerous. 
The smallest of the ova that I have examined are something less in length than 
