THE DEVELOPMENT OF STATOBLASTS. 



53 



.m 



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 

 jjlace. Those of 



the superficial M /^ 



layer assume the 

 appearance of 

 ordinary embry- 

 onic cells and 

 become applied 

 to the ectoderm 

 (Fig. 27, ec), thus 

 thickening the 

 layer of meso- 

 dermal elements 

 (?«) 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 miicedo (after Braem). A, on the lower side of the 

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

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

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

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

 with nuclei ; cc, ectoderm ; 7.s, 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. 



