XVII 



PEOTOCHOEDATA 



571 



blastula in having thicker walls. Further, it is composed of taUer 

 cells than the latter blastula. In both cases, however, it is obvious 

 that we have to do with indeterminate cleavage ; nothing even dis- 

 tantly suggesting the spiral cleavage of Annelida is to be seen. 



The blastula stage had been aheady attained by the vigorous eggs 

 when the material came into Heider's hands. On the following day 

 the blastulae became hemispherical, by the flattening of one side, 

 as do Echinoid blastulae, and the gastrula stage was attained 

 by the invagination of this flattened surface. The archenteron did 

 not completely fill up the blastocoele, since, coincidently with its 



ax' 



Fig. 415. — Early stages in the development of Balanoglossus dmigerus. (After Heider.) 



A, Blastula (about one day old). B, incipient gastrulation (one and a half days old). C, gastrala 



(two days old). D, formation of the anterior coelom. hlf, blastopore ; coe', anterior coelom. 



formation, the emlryo greiv in length. The blastopore was at first 

 wide, but became reduced to a narrow pore and finally closed altogether; 

 but the endodermic sac remained connected with the ectoderm at the 

 spot where the closure took place, and here, at a slightly later stage, 

 the anus was formed ; so that we may say that in Balanoglossus clavi- 

 gerus the blastopore becomes the anus, but for a brief period it is closed. 

 The front end of the somewhat elongated embryo then broadened 

 out, and the whole organism became flattened so as to have broad 

 dorsal and ventral surfaces and narrow sides. The front portion of 

 the archenteron became separated from the rest by a groove, and so 

 constituted the anterior coelomic vesicle, the rudiment of the pro- 

 boscis coelomic cavity of the adult (Fig. 415). The cells forming the 



