590 EMBRYONIC DEVELOPMENT OF ASCIDIANS. 



sinuses, which, are mere channels not having proper walls of their 

 own), of which some ramify over the Respiratory sac, hranching 

 off at each of the passages between the oval slits, whilst others are 

 first distributed to the Stomach and Intestine, and to the soft 

 surface of the Mantle. All these reunite so as to form a trunk, 

 which passes to the peduncle and constitutes the returning branch. 

 Although the Circulation in the different bodies is brought into 

 connection by the common stem, yet that of each is indepen- 

 dent of the rest, continuing when the current through its own 

 footstalk is interrupted by a ligature ; and the stream which 

 returns from the branchial sac and the viscera is then poured 

 into the posterior part of the heart, instead of entering the 

 peduncle. 



456. The Development of the Ascidians, the early stages of 

 which are observable whilst the Ova are still within the cloaca of 

 the parent, presents some phenomena of much interest to the 

 Microscopist. After the ordinary repeated segmentation of the 

 yolk, whereby a ' mulberry mass ' is produced, a sort of ring is 

 seen encircling its central portion ; but this soon shows itself as a 

 tapering tail-like prolongation from one side of the yolk, which 

 gradually becomes more and more detached from it, save at the 

 part from which it springs. Either whilst the egg is still within 

 the cloaca, or soon after it has escaped from the vent, its envelope 

 bursts, and the Larva escapes ; and in this condition it presents 

 very much the appearance of a Tadpole, the tail being straight- 

 ened out, and propelling the body freely through the water by its 

 lateral strokes. The centre of the body is occupied by a mass of 

 liquid Yolk ; and this is continued into the interior of three pro- 

 longations which extend themselves from the opposite extremity, 

 each terminating in a sort of sucker. After swimming-about for 

 some hours with an active wriggling movement, the Larva attaches 

 itself to some solid body by means of one of these suckers: if dis- 

 turbed from its position, it at first swims about as before ; but 

 it soon completely loses its activity, and becomes permanently 

 attached; and important changes manifest themselves in its inte- 

 rior. The prolongations of the central Yolk-substance into the 

 anterior processes and tail are gradually drawn back, so that the 

 whole of it is concentrated into one mass ; and the tail, now con- 

 sisting only of the gelatinous envelope, is either detached entire 

 from the body by the contraction of the connecting portion, or 

 withers, and is thrown-off gradually in shreds. The shaping of 

 the internal organs out of the Yolk-mass takes-place very rapidly, 

 so that by the end of the second day of the sedentary state the 

 outlines of the branchial sac and of the stomach and intestine may 

 be traced ; no external orifices, however, being as yet visible. The 

 pulsation of the heart is first seen on the third day, and the forma- 

 tion of the branchial and anal orifices takes-place on the fourth ; 

 after which the ciliary currents are immediately established through 



