2l6 THE ASCWIANS. 



perhaps two days' duration, is on the point of fixing itself 

 to a foreign object by means of the sticky secretion of its 

 three adhering papillae. 



This larva possesses features which we have not yet 

 considered. Let us give our attention in the first place 

 to the tail. 



Vaciwlisation of the Notochord. 



The vacuolisation of the notochordal tissue, which was 

 described above for Amphioxus, has already proceeded to 

 such an extent that there is no longer any trace of cellu- 

 lar structure in the centre of the notochord. It is entirely 

 filled with a perfectly colourless substance, probably of 

 gelatinous consistency, while the nuclei have been dis- 

 placed entirely from the centre and can be seen to lie 

 closely pressed against the dorsal and ventral sides of the 

 sheathing membrane of the notochord (Fig. 105 A). 



There is one respect in which the above vacuolisation 

 of the cells of the notochord differs considerably from 

 the corresponding process in Amphioxus and the higher 

 Vertebrates. 



Whereas in the latter forms the vacuoles appear inside 

 the individual cells, — in other words, are intracellular, — in 

 the Ascidian tadpole they occur between the cells, and are 

 therefore intercellular. This was first made out by 

 Kowalevsky, and can readily be observed. (Cf. Fig. 102.) 

 The intercellular spaces separate the cells which were 

 previously fitted accurately together, end to end, and, 

 gradually increasing in size, they eventually flow together 

 and so constitute a continuous space, while the cells with 

 their nuclei become thrust aside. 



Assuming that the vacuoles contain a more or less fluid 

 substance secreted by the protoplasm of the cells, the 



