EARLY DEVELOPMENT OF A STARFISH 463 



every instance, however, the distal wall of the archenteron is con- 

 tinuous, and the evidence is perfect that this anterior vesicle is 

 entirely formed from mesenchyme and not as an outpouching 

 of the archenteric wall. 



By the time that the anterior vesicle has attained a diameter 

 equal to that of the archenteron the mesenchyme migration 

 from the apical plate is at its height. Many cells in sections are 

 seen to have become entirely free, while others are in the act of 

 slipping past what are to become the final ectodermal elements, 

 and are making theu' way into the blastocele. In several larvae 

 they come in contact with the anterior vesicle (fig. 1), and cover 

 it like a roof. Where the mesenchyme is more scattered, a 

 relatively small number of cells rest upon the vesicle, while the 

 others pass around it, especially in the region of the future 

 oesophagus. 



Beyond this stage in the development, the walls of the anterior 

 vesicle usually disappear, the mesenchymal elements separating 

 and migrating to other regions. In the oldest stage where the 

 vesicle has been found to exist the stomodaeum had completely 

 formed, and the enterocele pouches had severed their connec- 

 tion with the gut. The vesicle in question showed sHght signs of 

 disintegration on its posterior face, but, as figure 4 indicates, it 

 is fairly complete, and has a diameter approximately equal to 

 that of the middle section of the body. Here it is a question 

 whether it has a complete wall next to the gut, but there is no 

 doubt whatever about its lack of communication with the 

 digestive tract. 



Surface views under fairly high magnification show the wall of 

 the vesicle to be made up of a relatively small number of cells. 

 These possess highly ramified pseudopodial processes which 

 appear to form an extremely delicate granular meshwork. 

 Whether the meshes are bridged by a non-granular ectoplasm 

 or by some intercellular material has not been determined, but 

 it is evident that the bounding wall, as a whole, is fairly com- 

 plete, since neither the pseudopodia of other cells nor minute 

 granules which appear to have escaped from some of the apical 

 cells make their way into the cavity of the vesicle. In some 



