XCV111 INTRODUCTION. 



What, then, are the extent and significance of the 

 change which takes place in the larva after its fixation ? 

 It has generally been held that in most cases the complex 

 organization which characterizes it in its free condition 

 suffers complete histolysis after its attachment, and is 

 replaced by a mass of structureless material, inclosed by 

 a membranous envelope. This was the view taken by 

 Schneider in his account of the remarkable larva of Mem- 

 branipora pilosa *, and adopted by Nitsche, Joliet, and 

 others. According to the later observations of Barrois, 

 however, this is by no means a correct representation of 

 the actual fact. What really occurs is not an utter dis- 

 solution of the larval organism, but merely loss of a 

 special larval organ, which disappears after fixation, as 



Fig. ilii. 



Ci'U of Schizoporella shortly after fixation. 



C. Cell. gr. Granular mass, result of liietolysis of the corona. 



B P. Rudiments of polypide. 



the tail of the Ascidian larva undergoes histolysis when 

 it becomes attached. The organ which, according to 

 Barrois, is the starting-point of the histolysis amongst 



* Sitz.-Bericht. d. Gesellschaft nntxirf. Ffeund. zu Berlin, 1868. 



