572 



INVERTEBRATA 



CHAP. 



wall of this sac lost their cylindrical form and became flattened, and 

 sent out pseudo-podia by which the coelomic sac was anchored, so to 

 speak, to the ectodermal walls of the body. 



The anterior coeloni now sent out a prolongation which reached 

 the anterior pole of the embryo ; at this point an apical plate was 



coe 1 



FIG. 416. Later stages in the development of Balanoglossus clavigerm. (After Heider.) 



A, formation of the apical plate. B, formation of mouth anil of pore-canal. C, mouth anus and 

 water pore formed, a, anus; ap, apical plate; fuel, anterior cni'lom ; int. intestine; <>, mouth; oes, 

 endodermal section of oesophagus ; p.c, pore-canal ; rf, stomach ; *ttnn, stoniudurum ; cy/i, 



formed by a thickening of the ectoderm, and the cells forming this 

 thickening developed long stiff cilia, so that in this way a typical 

 apical plate was formed. From the coelomic sac also a dorsal pro- 

 jection was developed which grew backwards and fused with the 

 ectoderm, and here, somewhat later, an opening was effected which 

 became the proboscis-pore or water-pore of the adult (w.p, Fig. 

 416, C). 



