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. 1 «), with a round, smooth and opake 

 body in its centre (fig. 1 b). This membranous case (chorion), 

 which I shall designate the case-membrane , is of a circular or 

 oval form, is larger than what is sufiicient to contain the opake 

 body within it, and its walls appear to be composed of at least 

 two distinct laminse*. The opake body within is of a round form, 

 and is chiefly composed of minute cells (nuclei), intermixt with 

 a structureless substance which I suppose to be semifluid, and 

 the whole is inclosed in an external transparent membrane (vitel- 

 line membrane t). I shall restrict the term ovum 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 thejE^o/?5 were nearly of the same size, while those 

 of the Doris tuber culata were considerably larger and those oiDoto 

 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 KoUiker 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 quadrilineataj Doto coronata, Doris 



* I observed these lamiiiEe separated from each other in some parts by a 

 distinct interval in several ova of the spawn of the Eolis. 



f Fig. 2 is a diagram showing the external or vitelline membrane imper- 

 fectly filled with its contents. 



I 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. . 



