DEVELOPMENT OP FLUSTRELLA HISPIDA. 461 



between the ectoderm and endoderm, and Braem considers 

 that this tissue may represent the mesoderm, and that it is 

 possibly derived from the initial cells separated off from the 

 original four large endoderra cells. 



Calvet (8), in his general account of the embryology of 

 Cheilostoraes and Ctenostomes, makes the general statement 

 that segmentation is equal and regular up to the thirty-two- 

 cell stage ; but, from the above description of the pi-ocess in 

 Flustrella hispida, this is obviously not invariably the 

 case. Calvet describes the formation of endoderm as taking 

 place in a manner similar to that in which it originates in 

 Flustrella, and he arrives at the conclusion that the 

 endoderm arises partly by endocytulation, partly by plann- 

 lation. 



The endoderm, therefore, appears to originate in a similar 

 manner in Flustrella and in the few other Bryozoa in 

 which its formation has so far been investigated. 



The Development of the Larval Organs. 



The formation of mesendoderm being completed, the first 

 traces of larval organs soon appear in the shape of a two-fold 

 invagination of the oral ectoderm, and this is followed soon 

 after by a third invagination at a point anterior to the 

 previous ones. Both ectoderm and mesendoderm cells have 

 by this time begun to lose their definite cell outlines. 



Ectodermic Organs. — The internal sac is the first 

 organ to be formed. It arises as a median invagination 

 of the oral ectoderm ; and in PI. 25, fig. 62, it is seen as an 

 elliptical space {1.8.) communicating with the exterior by 

 means of a narrow opening. The cells lining this sac are short 

 and flat, and their nuclei lie close to the periphery. These 

 cells are seen to contain large globules of a substance closely 

 resembling the yolk spherules in their general appearance and 

 in their reactions to staining reagents ; and drops of this sub- 

 stance are also found to be exuding from many cells (PI. 25, 

 fig. 62 g.). This substance subsequently disappears entirely 



