316 A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 



the meanwhile, however, the superficial layer of the axis of the 

 tail, which is composed of smaller cells, appears to have become 

 metamorphosed into a muscular layer consisting of longitudinal 

 fibres, by whose energy those active movements of the tail ex- 

 hibited by the young larvae after birth are performed. At 

 present these movements are manifested only by the occasional 

 twitches of the tail, which become more and more frequent in 

 the last period of larval development. In consequence of these 

 twitchings, the vitelline membrane eventually becomes torn, and 

 thus, towards about the thirtieth hour after fecundation, the 

 larva escapes. 



One point must not escape notice ; with regard to those two 

 dark pigment spots which have been observed on the back of 

 the larval embryos, but which have been, as it seems to me, too 

 hastily taken for eyes. At first we see only a single pigment spot 

 exactly in the middle line of the back. Behind this and more 

 laterally a second, larger one, soon becomes visible (PI. XII. B. 

 fig. 1. e). I have never succeeded in discovering any refractive 

 medium in these pigment spots ; but I could, as little, satisfac- 

 torily interpret the optical expression of the immediate neigh- 

 bourhood of these spots, which I have endeavoured to repro- 

 duce truly in the figure just cited. This much is certain, that 

 both pigment spots, which in the larva remain always separate 

 from one another (see fig. 2), approach quite closely in the 

 course of metamorphosis and, during the development of the 

 young Ascidian, continue to appear for a long time as an appa- 

 rently single mass, lying close below the nervous ganglion. In 

 the end, however, this mass breaks up into the two original 

 portions, or even into many pieces, and so passes into the 

 current of the blood, in which they may be seen driven up and 

 down for a time, until at last they are wholly dissolved and dis- 

 appear. This long continuance of the pigment spots, extending 

 far beyond larval life, appears to me not to be in accordance 

 with the function ascribed to them. For the present, therefore, 

 their true import remains doubtful. 



3. Larva (see fig. 2). — The body of the hatched larvae is 

 elongated, has two slightly convex, lateral surfaces and is pro- 

 vided at its anterior extremity with three very short processes, 

 as it would seem excavated into suckers. Two of these lie 



