480 LECTURE XX. 



which severally open a passage constituting, the one an oral, the other 

 an anal, orifice. The subdivided germ-mass, which now begins to 

 be rapidly metamorphosed into the special tissues, also acquires a 

 distinct tunic, which soon separates itself from the thick and gela- 

 tinous external integument. The quadrifid orifice of the branchial 

 sac is first formed upon the internal tunic. The contour of the great 

 respiratory pharynx can next be discerned, and the constriction of the 

 sac opposite to the mouth, which indicates the oesophagus. About 

 the same time may be seen the outline of the anal oi-ifice upon the 

 internal integument ; then the opaque yellow tunics of the dilated 

 stomach, and the reflected intestine appear ; and below these parts 

 the pulsations of the large transparent vasiform heart render that 

 organ conspicuous. Around each external orifice some mammilloid 

 processes bud out, which first lengthen, then become lost in the thick- 

 ening integument. The eye-speck continues for some little time, and 

 is situated in the middle of the nervous collar. At the base of the 

 abdomen the opalic concretionary body appears, to which the heart is 

 subsequently attached, and which is provided with vibratile cilia. 



The whole of the viscera included by the smooth integument have 

 been observed to rotate in the cavity formed by the thick gelatinous 

 tunic, to which the visceral mass again becomes attached by the ad- 

 hesion of the muscular tunic at the branchial and anal orifices, and 

 by the establishment of corresponding orifices in the integument. 



Savigny was of opinion, that the ovum of the compound Ascidian 

 contained the germs of all the individuals composing the characteristic 

 groups in the mature aggregate animal, and that their development 

 was simultaneous. In one sense, doubtless, the ovum contains the 

 germs of all the future individuals developed by gemmation, in so far 

 as a portion of the germ-mass is retained unchanged in the body of 

 the first developed individual ; but the cell-progeny of the primary 

 germ-cell constituting that germ-mass, are not simultaneously deve- 

 loped : nor does any development begin until the first individual is 

 completed, fixed, and nourished by the actions of its proper digestive 

 apparatus. Thus stimulated and strengthened, the second mode of 

 reproduction, namely, that by gemmation, is superinduced upon the 

 young Ascidian, after the foregoing development from the impreg- 

 nated ovum, which offers an interesting analogy to the phenomena 

 presented by the polype-larva of the Medusa. The individuals 

 formed by the gemmation of the primary bud of the young Ascidian, 

 instead of being detached, are retained ; the process of gemmation 

 being regulated so as to produce the characteristic pattern in 

 which the different individuals are grouped in the mature compound 

 animal. 



