380 Dr. Reid on the Development of the 
and connected together, and to the surface of the object upon which 
they were placed, by a transparent structureless substance. This 
chord when arranged in this manner made up a larger chord from 
three-twelfths to four-twelfths of an inch in diameter and several 
inches in length, which formed several coils, some of which crossed 
or were superimposed upon others. The structure of this chord 
was the same as that of the Dendronotus arborescens. 
When the ova are examined under the microscope soon after the 
extrusion of the spawn, each is seen to consist of a thin transparent 
membranous case (Pl. X. fig. 1a), with around, smooth and opake 
body in its centre (fig. 1 6). This membranous case (chorion), 
which I shall designate the case-membrane, is of a circular or 
oval form, is larger than what is sufficient to contain the opake 
body within it, and its walls appear to be composed of at least 
two distinct lamine*. The opake body within is of a round form, 
and is chiefly composed of minute cells (nuclei), mtermixt with 
a structureless substance which I suppose to be semifluid, and 
the whole is inclosed in an external transparent membrane (vitel- 
line membranet+). I shall restrict the term ovwm to this opake 
body inclosed in the case-membrane{. The ova of the Doris bi- 
lamellata vary in size from about 1-250th to 1-280th of an inch in 
diameter, those of the Holts were nearly of the same size, while those 
of the Doris tuberculata were considerably larger and those of Doto 
coronata smaller than this. The minute cells (nuclei) composing 
the greater part of the vitelline mass are of a round or oval shape 
(fig. 3), vary in size in the Doris bilamellata from 1-6000th to 
1-9000th—the greater number being from the 1-7000th to 
1-8000th—of an inch in diameter, and no nucleoli were observed 
in their interior. A very great number of these ova were examined 
when subjected to very different degrees of pressure, and their 
structure appeared to be uniform, presenting no differences at 
different parts, and entirely composed of the materials we have 
described. I endeavoured to discover a clear cell in the centre 
of the vitelline mass, similar to that described by Kolliker and 
Dr. Bagge as existing in the ova of different species of Ascaris, 
and which plays so important a part in the cleaving of the yolk ; 
but if such a cell exists, it escaped my notice from the opacity of 
the yolk. No evidence of the presence of this cell was obtained 
in any of the subsequent changes through which the ovum passed. 
In the spawn of the Polycera quadrilineata, Doto coronata, Doris 
* I observed these laminz separated from each other in some parts by a 
distinct interval in several ova of the spawn of the Zolis. 
+ Fig. 2 is a diagram showing the external or vitelline membrane imper- 
fectly filled with its contents. ; 
t No doubt the case-membrane is an accessory part of the ovum, and it 
is for convenience sake that I use the latter term in this restricted sense. 
