1222 



TUNICATA. 



When the ova, lodged in the marsupial 

 pouch or cloaca, have arrived at a rather more 

 advanced period of their development, the vi- 

 tellus loses the mulberry appearance it had 

 but lately put on, and, if compressed between 

 two slips of glass, it is seen to be wholly com- 

 posed of minute globules or granules of dif- 

 ferent sizes. The ovum is soon a little flat- 

 tened, and the yolk appears to be concentrated 



Fig. 



towards the middle, forming an ovoid mass of 

 a deep yellow colour, surrounded by a some- 

 what large border of a clearer tint. This mar- 

 ginal portion of the yolk is condensed in its 

 turn, and although at first sight appearing to 

 constitute a sort of ring, becomes a long taper- 

 ing prolongation, which encircles the central 

 part of the yolk, adhering by its base, and 

 having its pointed extremity free (Jig- 784. A). 

 784. 



j^arvai asciaia taieiy oorn, magnified aoout _ _ 



forming the proper tunic of the body of the larva ; b", processes terminating in suckers, and serving 

 ix the animal ; a, tail formed by a prolongation of the integuments, and enclosing a tubular ap- 



The Development of the Larva of Amaroucium proliferum. (After Milne- Edwards.') 



A. An ovum the incubation of which is far advanced, magnified about 30 times. The tail, (&') is becoming 

 distinct from the trunk (6) ; and two lobules begin to appear on the anterior extremity of the latter (&'/). 



B. An ovum arrived at the full term of incubation, magnified about 30 times, a, the tegumentary portion ; 

 b>, caudal portion ; b", b'", anterior appendages. 



c. Larval ascidia lately born, magnified about 25 times, a, tegumentary portion ; b, sac enclosing the yolk 



to fix 



pendage of the vitelline sac. 



D. The same larva, observed some hours after having fixed itself, magnified about 20 times, b", traces of the 

 anterior processes ; b>, vitelline prolongation of tail nearly absorbed, and the central sac, enclosing vi- 

 tellus, is spherically contracted. 



E. The same larva about 20 hours after fixing, magnified about 25 times. The caudal elongation of the 

 internal tunic (containing the vitelline matter) has entirely disappeared, and this tunic (6) has taken 

 the form of an ovoid sac, slightly contracted in the middle ; a pale yellow circle (e?) at the anterior ex- 

 tremity surrounds a spot that will become the mouth, and posteriorly another, clear, spot (c) appears, 

 in which the heart will be developed. 



F. The same larva seen at the end of the second day of its sedentary state, magnified about 25 times. 6. tunic 

 proper ; c, pericardium spot ; e, branchial sac, beginning to be developed ; /, thoracic sinus ; g, cloaca ; 

 /, stomach ; m, intestine, full of fsecal matter ; a, vestige of tail, of which a part only is figured. The 

 external orifices are not yet formed. The development of this individual was much more rapid than 

 visually is the case. 



G. Another individual, about 8 days old, magnified. Animal completely reversed in the interior of the 

 tegumentary envelope ; d, oral orifice ; i, anal orifice ; /, stomach ; m, intestine ; o, heart. 



H. Individual about 20 days old, magnified (lying with its anterior extremity to the right), a, tegu- 

 mentary envelope ; m, faecal matter in the intestine. 



In the course of incubation, the ovum in- 

 creases in size, becoming flatter and more 

 oval. The vitelline mass becomes more com- 

 pact, and its surface denser; and the latter 

 seems to be organised into a membrane, dis- 

 tinct from the yolk beneath. The two por- 

 tions of the ovum, before described, become 

 more and more separate; that which occupies 

 the centre of the ovum becomes ovoid, and 

 knotty at one of its extremities; near the other 

 extremity, which is continuous with the mar- 

 ginal portion, are seen one or two blackish 

 minute points. This ring-like portion is now 



which also it is distinctly divided a little in 

 front. Lastly, the whitish substance sur- 

 rounding the vitellus, and constituting the 

 tegumentary mass, increases considerably in 

 thickness. 



When the ova more nearly approach ma- 

 turity, the tail-part of the vitellus is shortened, 

 and its central part, or body of the embryo, is 

 more and more condensed. Its anterior ex- 

 tremity becomes lobulated and encircled with 

 a series of five cylindrical processes, which have 

 a divergent arrangement, and advance towards 

 the border of the egg. Three of these ap- 



seen to be a caudal prolongation, too short pendages terminate in a kind of button, and 

 completely to encircle the central part, from the intervening two are tapering anteriorly. 



