132 Papers from the Marine Biological Laboratory at Tortugas. 



pluteus, and it is not until the fifth day that the arms begin to push out. 

 These facts are of interest when compared directly with those regarding the 

 development of Toxopneustes. 



The processes are here shown in parallel. The hours mentioned indi- 

 cate hours after fertilization. 



Cidaris. 



Hours. 

 Blastute (swimming) i6 to i8 



Gastrulae (beginning) 20 to 23 



Mesenchyme 23 to 26 



Chromatophores 44 



Skeleton (beginning) 72 to 73 



Pluteus 120 



Toxop}ieustes. 



Hours. 



Blastulae (swimming) 5.5 



Mesenchyme 8 



Gastrulae (beginning) 9 



Chromatophores 15 to 16 



Skeleton (beginning) 15 to 16 



Pluteus 24 



The earliest striking difference between the two forms is in the time 

 and place of mesenchyme formation, Cidaris in this respect resembling the 

 crinoids more closely than the echinoids. 



No mesenchyme is formed until the archenteron has pushed well into 

 the blastocoele (fig. i). The cells then push out from the end and from 

 just behind the end of the archenteron. During the next few hours the 



Fig. I. — Optical section, Cidaris gastrula, showing beginning of mesenchyme formation. 

 Fig. 2. — Optical section, later Cidaris gastrula. 41 hours. 



23 hours. 



migration of cells from the archenteron continues, while the cells within the 

 blastocoele increase in number by division. A network formed by fine 

 protoplasmic processes, as in other echinoids, is formed (fig. 2). 



In Toxopneustes, it will be recalled, the micromere end of the blastula 

 becomes flattened and the primary mesenchyme cells push into the blasto- 

 coele before gastrulation begins (fig. 3). This mesenchyme is first scattered 

 throughout the blastocoele, the greater number of the cells remaining, how- 



