Early Influence of Spermatozoan Characters of Echinoid Larvce. 133 



ever, at the posterior pole. When the archenteron pushes in they lie as 

 a ring about it, later becoming aggregated in two groups, one at each side 

 of the base of the archenteron; in each group the characteristic skeletal 

 spicules arise. The secondary mesenchyme is given off from the end of the 

 archenteron after this has grown well toward the anterior end of the blastula 

 (fig. 4). 



Fig. 3- — Optical section, Toxopneusles blastula, showing beginning of primary mesenchyme formation. 



8 hours. Drawn from preserved material. 

 Fig. 4. — optical section of Toxopneusles gastrula, showing formation of secondary mesenchyme and 



of skeleton. Chromatophores in blastocoele and wall. 41 hours. 

 Fig. S. — Young Cidaris pluteus, showing character and position of early skeleton. 73 hours. 

 Fig. 6. — Cidaris pluteus. 6 days. 



In the case of Cidaris, after the formation of the primary enterocoele and 

 the dorsal water-tube (50 hours) , the gastrula begins to elongate and to 

 become broader at the posterior end. When the primary enterocoele sepa- 

 rates into right and left enterocceles (the left remaining in connection with 

 the water-tube (55 hours), two ciliated bands make their appearance, the 

 posterior band about one-third the length of the body from the posterior 

 end of the body, the preoral band somewhat less than one-third the length 



