AND DEVELOPMENT OF PYROSOMA. 229 



of it sufficient to give colour, is exactly tliat of the contents of the germinal vesicle ; the 

 diameter of the semicii'cular portion is hut very slightly greater than that of the germinal 

 vesicle in its later stages ; and finally, the minute bodies which occupy the centre of each 

 component corj)uscle of the membrane are not a little similar in character to the small 

 spheroidal particles which appear upon the contents of the germinal vesicle durino- 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 

 siu'face 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 of 

 the spermatozoa, that I have been frequently tempted to regard them as such, and the 

 more so, as in this stage the duct looks shrunken and shrivelled, and contains but very 

 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 ; btit as, in spite of long search, the total number which I have found in this 

 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, inioMchthe blastoderm 

 rajiidlij increases, and becomes segmented into the rudiments of five 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 mtist 

 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 in 

 the mid-atrivun, which, in its later stages, it fills. To arrive at this position it must 

 necessarily break through the wall of the atrium or atrial timic, 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 heemorrhage. Uow- 

 ever, the zooids in which the detached ovisacs have attained a large size appear to be 

 in as good condition as any of the rest. 



Henceforward I shall speak of this complex body, composed of the ovisac and the 



