development of flustrella hispida. 467 



The Deget^eeating Larva. 



Shortly after the stage wLicli has been briefly described 

 above, the degeneration of the larval organs commences. 

 The initiation of this process is shown in PI. 2b, fig. 66. 



The internal sac becomes enormously thickened, and its 

 lining ectoderm highly modified, especially near the opening 

 of the sac to the exterior, where it now assumes a granular 

 character. The pharynx gradually loses its cellular structure ; 

 the pyriform organ is at this stage still fully developed, as 

 are also the aboral organ and neuromuscular tract. The 

 ciliated crown, as has already been stated, consists at this 

 stage of a single ring of large cells. The stomach has prac- 

 tically vanished, its position being marked only by a number 

 of scattered yolk spherules and of nuclei lying between the 

 internal sac and the aboral ectoderm. 



Origin of the Mesoderm. — Among the above-men- 

 tioned scattered elements of mesendodermic origin, and 

 apparently developed from them, occur fibres (PI. 25, 

 fig. 66, M), which are presumably muscular in nature. Others 

 of these supposed muscular fibres occupy the former position 

 of, and are probably developed from, the lateral bands of 

 tissue previously noted as budding off from the stomach 

 It is this mesendodermic mass of yolk spherules, nuclei, and 

 fibres which Prouho (20) regards as representing the meso- 

 derm in Flustrella hispida. He maintains that the 

 mesoderm occurs as a distinct layer of cells lying beneath 

 the aboral ectoderm, generally thickened at the aboral pole; 

 that a similar membrane overlies the internal sac, and also 

 that all the muscular elements of the larva are of mesodermic 

 origin. But, as has already been shown, it is quite impos- 

 sible at any early larval stage to differentiate the mesoderm 

 from the general endodermic mass, and Prouho's so-called 

 '' mesoderm " is, therefore, undoubtedly not simply mesoderm, 

 but endoderm, or perhaps rather mesendoderm, since it is, of 

 course, possible that in this endodermic mass lie enclosed tli6 

 elements of the future mesoderm, from which the muscles are 



