THE DEVELOPMENT OP STATOBLASTS. 



53 



layer under the ectoderm, are of importance in the development of 

 the mesoderm-layer (Fig. 27 A, m) of the young colony. 



The completely formed statoblasts which, after the disintegra- 

 tion of the parent colony, become free, are not at once capable of 

 regeneration. The capacity for further development, as a rule, 

 appears only after they have been frozen, or after a long period 

 of rest when air has been excluded (BRAEM). A higher temperature 

 or contact with air stimulates the statoblasts, which are then capable 

 of germinating, to further development. 



A transformation of the cells of the inner yolk-mass first takes 

 place. Those of 

 the superficial 

 layer assume the 

 appearance of 

 ordinary embry- 

 onic cells and 

 become applied 

 to the ectoderm 

 (Fig. 2 7, ec), thus 

 thickening the 

 layer of meso- 

 dermal elements 

 (m) mentioned 

 above as lying 

 below the ecto- 

 derm. This me- 

 sodermal layer 

 consequently 

 soon becomes a 

 continuous epi- 

 thelium. The 

 first rudiment 

 of the primary 

 polypide consists 

 of a rounded ec- 

 todermal thick- 

 ening (germ-disc, 

 Fig. 27 A, P ) 

 which develops 



in the middle of that side of the statoblast which, when floating, is 

 turned downwards, i.e., at the point at which the circumcrescence 



FIG. 27. Two ontogenetic stages of the germinating statoblasts of 

 Cristatella mucedo (after BRAEM). A, on the lower side of the 

 statoblast (that turned upwards in the figure) the polypide- 

 rudiment (p) can be seen in the form of a germ-disc. B, the 

 polypide-rudiment in a more developed condition, a, anus ; an, 

 intestinal rudiment ; c, cuticular shell (disc) ; d, yolk substance 

 with nuclei ; ec, ectoderm ; ks, budding zone of the future 

 polypide ; m, mesoderm-layer ; n, ganglionic invagination ; o, 

 rudiment of the oesophagus ; p, germ-disc ; od, upper spines ; 

 sr, float ; ud, lower spine. 



