Hybridization of Echinoid.s. 9 



In some sections of embryos of an age of 73 hours the mouth opening 

 does not seem to be established; in others the opening is present. 

 It is therefore safe to say that the mouth opens about the beginning 

 of the fourth day. The first appearance of skeletal spicules is about 

 this time, these spicules arising in groups of mesenchyme cells at 

 about the level of the outer ends of the posterior ciliated bands. As 

 these rods increase in length, the sides of the embryo grow out to form 

 what are probably the post-oral arms (figs. 4, h and c), the fenestrated 

 rods in their growth keeping pace with the growth of the arms. 



The cultures were kept under observation until the end of our stay 

 at Montego Bay (two weeks). During this time no essential change 

 in the form of the larva took place. The skeletal rods increased in 



Fig. 3a, Optical section living Cidaris gastrula; somewhat flattened by 

 pressure of coverslip; 41 hours; enterocoele forming; mesen- 

 chyme cells abundant in blastoccele. X 140. 



36, Section of blastula oi Lytechinvs; 6 hours; iroux fixed material. 

 X 285. 



3c, Section of blastula of Lytechiniis with mesenchyme cells 

 proliferating; 7 hours; from fixed material. X 285. 



3d, Optical section living gastrula of Lytechinus; 20 hours. X 285. 



length and the inner ends of these rods assumed a more definite 

 association as a framework at the sides of and ventral to the stomach 

 (fig. 56). In the most advanced specimens that have been studied 

 the fenestrated rods show as many as 14 openings, and there has been 

 a pronounced increase in the length of the antero-lateral rods. The 

 body-rods extend posteriorly on each side of the stomach, while the 

 ventral transverse rods have grown toward the median ventral line, 

 although there has as yet been no contact or union between the ends 

 of the rods of opposite sides. 



Comparison with other Investigations of CmARis. 



Cidaris tribuloides, as may be seen from the foregoing description, 

 is quite unlike the more familiar echinoids in the time and place of 



