AND DEVELOPMENT OF PYROSOMA. 229 
of it sufficient to give colour, is exactly that of the contents of the germinal vesicle ; the 
diameter of the semicircular portion is but very slightly greater than that of t lie germinal 
vesicle in its later stages ; and finally, the minute bodies which occupy the centre of each 
component corpuscle of the membrane are not a little similar in character to the small 
spheroidal particles which appear upon the contents of the germinal vesicle during the 
latest stages of its existence. 
Putting all these circumstances together, I venture to express the belief that this 
membrane, which the further progress of development proves to be the blastoderm out of 
which all the parts of this embryo take their rise, results from the metamorphosis of the 
contents of the germinal vesicle ; and that the curved contour which lies towards the upper 
end of the duct is, in fact, the contour of that side of the germinal vesicle which first 
becomes filled with the yellow deposit. 
Thus far, I feel little difficulty in interpreting the appearances presented ; but if the 
surface and the immediate edges of the blastoderm are examined with great care, minute 
rod-like bodies will be seen scattered about, so similar in form and size to the heads oJ 
the spermatozoa, that I have been frequently tempted to regard them as such, and the 
so. as in this staire the duct looks shrunken and shrivelled, and contains but very 
more 
o 
few, if any, remains of the plug of spermatozoa so conspicuous previously. 
In this stage, each of the blastoderms which I have examined has presented these 
appearances; but as, in spite of long search, the total number which I have found in thi< 
state does not exceed four, I do not feel myself in a condition to pronounce positively 
upon the nature of the bodies in question. 
Seventh Stage. Ovisacs from &th to &th of an inch in diameter, in which the blastoderm 
rapidly increases, and becomes segmented into the rudiments office zooids. 
Up to this stage the ovisac lies within the sinus system of the parent, which, as I have 
already pointed, out, becomes accommodated to its increased dimensions, partly by the 
thrusting of the atrial tunic into the cavity of the atrium, but, to a much greater extent, 
by the formation of a chamber in the test, in consequence of the extension outwards of a 
diverticulum of the outer tunic. In the recent condition, the blood of the parent must 
circulate in the narrow space left between the walls of the ovisac and those of its contain- 
ing chamber ; and it seems reasonable to suppose that the former imbibes into its interior 
a supply of nutritive material, which will contribute towards the subsequent development 
of the embryo. 
But during and after this stage, the ovisac bearing the embryo is to be found loose m 
the mid-atrium, which, in its later stages, it fills. To arrive at this position it must 
necessarily break through the wall of the atrium or atrial tunic, and through the duct 
which still connects it with that tunic. The latter process is easily intelligible, considering 
the very small relative size and delicacy of the duct ; but I confess I do not understand 
how the rupture of the atrial tunic can be effected without serious hemorrhage. How- 
ever, the zooids in which the detached ovisacs have attained a large size appear to be 
m as good condition as any of the rest. , 
Henceforward I shall speak of this complex body, composed of the ovisac and the 
