IV. i INITIAL STRUCTURE OF THE GERM 



205 



larva with mesenchyme and apical organ, but the archenteron is 

 solid. After the next phase eight cells the upper quartette 



FIG. 114. Cerebratulus. Larvae from upper quartettes of the 8-cell 

 stage. A. Larva from whole quartette. B, C. The same, but one cell 

 was injured. D, E, F. Slightly older, from complete quartette. (After 

 Zeleny, 1904.) 



(1 a-l d) produces a larva with an apical organ and mesenchyme 

 but devoid of an archenteron ; the larva developed from the lower 

 quartette (A D} has an archenteron but no apical organ, while a 



A 



FIG. 115. Cerebratulus. Larvae from lower (vegetative) quartettes 

 of 8-cell stage. Note absence of apical organ, presence of large, hollow 

 or solid, archenteron. (After Zeleny, 19C4.) 



meridional half, comprising two macromeres and two micromeres, 

 develops both organs, though it is usually asymmetrical. 



In the 16-celled stage the isolated apical cells (1 a 1-1 d 1) will 

 give rise to a ciliated blastula with mesenchyme and an apical 



