408 RICHARD EVANS. 



rect — ihat the nuclei of the flagellated cells became completely 

 absorbed. The latter, on the other hand, become nuclei of 

 the young sponge, and this is true equally of the central vesi- 

 cular nucleus and of the numerous smaller nuclei belonging 

 to the flagellated cells. 



Simultaneously, as a rule, with the above changes in the 

 characters of the yolk bodies, and the small nuclei contained 

 in the plasmodial aggregations, the cytoplasmic bodies of 

 the groups in question lose their sharp outline. They spread 

 out and become irregular in shape and almost indistinguishable 

 from one another. By the time the nuclei have attained the 

 definitive structure of the nuclei of collar-cells, the internal 

 arrangement of the cytoplasm belonging to the various groups 

 of plasmodial aggregations may be described as being syncytial. 

 Spaces begin to make their appearance in the undifferentiated 

 cytoplasm (fig. 18), which soon develop into large cavities 

 (fig. 19), lined by cells possessing granular nuclei. Mean- 

 while the nuclei of the flagellated cells arrange themselves in 

 the rings of the cytoplasm, which are at first quite irregular 

 and ill-defined (fig. 19 a), and do not appear to consist of 

 individual cells ; but this syncytial condition soon passes away, 

 and the separate individual collar-cells make their appearance. 

 They develop collars and flagella at the time of separation 

 as free cells, and in this way the plasmodial aggregations give 

 rise to the flagellated chambers. The cell with vesicular or 

 granular nucleus which occupied the central position retires 

 outside the chamber, and takes no part in its formation. 



(b) Special Features of the Metamorphosis of the 

 Different Types of Larvse. 



The phenomena of the development common to all the 

 types having been described in the previous section, in the 

 following pages only those features which characterise the 

 metamorphosis of the fully developed types, namely, C and D, 

 will be specially considered. Eeference must be made also to 



