TUNICATA. 479 



still included in the ovarian mass, consists of the small central 

 germinal vesicle, of a granular vitellus, and a vitelline membrane. 

 In the progress of the ovum to the cloacal cavity, the yolk acquires 

 a deep yellow colour, the germinal vesicle disappears, and in its 

 place there is a nebulous speck upon the surface of the yolk. This 

 is doubtle€S the modified germinal vesicle, which has come to the 

 surface of the yolk to meet the impregnating influence, and has 

 undergone the changes by fissiparous multiplication, to which I have 

 so often had occasion to allude, Edwards has observed the contact 

 of the spermatic animalculae with the ova in the cloaca. 



The next stage which he records is the granular or mulberry 

 structure of the vitellus. The subdivided mass through which the 

 properties of the hyaline and fertilising principle have been diffused, 

 is next covered by what appears to be the expansion of the germ- 

 spot, or a propagation from that centre of nucleated cells, closely 

 pressed together into hexagonal forms, constituting what Herold 

 calls the " cambium " in the spider's ovum, and forming the basis of 

 the integument. A process of this integument then begins to extend 

 from a particular point, and, rapidly elongating, wraps itself like a 

 cord about the vitellus. This body, with its integument, next be- 

 comes condensed, and separates from the chord, which, retaining 

 only its basal attachment to the pellucid integument, forms the cau- 

 dal appendage. The integument {Jig. 181, a) increases in thickness. 

 The extremity of the yolk opposite the caudal attach- 

 ment developes a series of cylindrical productions. < j=- 

 Three of them have expanded extremities (6", b") 

 which increase in length ; whilst the other processes di- 

 minish, and finally disappear. A spiral filament is con- 

 tinued from the membrane of the vitellus down the 

 centre of the tail. 



In this state the embryo escapes from the ovum, gene- 

 rally while in the cloaca of the parent, but sometimes '- ' 

 after the egg has been expelled from the common central 

 outlet. The young animal immediately unfolds its tail, 

 and begins to swim hke the tadpole of the frog, which 

 it so much resembles in form. The three clavate cephalic 

 processes are the organs by which Edwards believes it 

 effects its final adhesion and settlement. When this has W 

 taken place, the tail shrinks, and is usually detached by Larraof 

 progressively increasing contraction at its base; — a p^^en*m? 

 kind of spontaneous fission. 



The sessile and adherent trunk now becomes the seat of an active 

 development : the integument is thickened ; the germ-mass becomes 

 elongated and divided by a circular constriction into two unequal parts. 



