Hyhridizaiion of Eddnoids. 7 



blastocoele from the flattened and thickened posterior pole of the 

 blastula in 8 hours; the process of gastrulation begins in 9 hours; 

 chromatophores appear in 15 to 16 hours; skeletal spicules appear in 

 15 to 16 hours; and the young pluteus stage is reach in 24 hours after 

 insemination; it may be reached in as short a time as 20 hours. 

 These facts are shown in parallel columns (p. 6) . The hours mentioned 

 indicate hours after insemination. The difference in sequence of 

 stages as well as the difference in rate of development is evident. 



As has been stated above, there is nothing unusual in the develop- 

 ment of Cidaris, aside from extreme slowness, until the stage when 

 mesenchyme formation might be expected. The blastulse (fig. la; 

 plate 3, fig. f) have a wall of rather uniform thickness. The cells 

 at the posterior end seem very sHghtly larger than those of other 



Fig. la, Optical section Cidaris blastula 16 hours old. Drawn from fixed 



and stained material. X 285. 

 16, Optical section Cidaris beginning gastrulation; 20 hours. Disivrn 



from fixed and stained material. X 285. 

 Ic, Optical section Cidaris gastrula, 22 hours; drawn from fixed and 



stained material. X 285. 



parts of the wall. A section of an 18-hour blastula (plate 3, fig. f) 

 shows that these larger cells are about to move or be forced inward. 

 The first indication of an archenteron appears in embryos of about 

 20 hours (fig. 16). In these, at the posterior end, there may be seen 

 a small, hollow plug of cells, extending into the blastoccele. The cells 

 forming this plug are quite characteristically rounded, standing out 

 distinctly from each other like the seeds in a blackberry. The cavity 

 of the archenteron, though shallow, is cylindrical and has a distinct 

 blastopore. The archenteron continues to grow forward into the 

 blastoccele, and 3 hours later has about doubled in length (figs. Ic, 2a; 

 plate 3, fig. g). The inner end is strikingly irregular in form, because 

 of the protrusion of cells from its surface. No mesenchyme cells 

 have as yet migrated from the wall of the archenteron, but sections 

 (plate 3, fig. h) reveal the fact that they are in process of withdrawal 

 at this time. The apparently loose contact of the cells of the arch- 



