Hyhn'dization of Eddnoids. 13 



the probability, although it does not absolutely establish the proof, 

 of the conclusion that the development of Cidaris is primitive. 



Observations on the Hybridization of Cidaris. 



At the time when these observations on the development of Cidaris 

 were made, March 1912, two other sea-urchins in the region were 

 breeding. These were Lytechinus (Toxopnensles) variegatus and 

 Tripneustcs (Hipponoe) esculenta. The fertilization of the egg of 

 Cidaris by either Lytechinus or Tripneustes sperm was easily accom- 

 plished. No preliminary treatment of the eggs was necessary. It 

 was possible to fertilize the eggs with either Lytechinus or Tripneustes 

 sperm immediately after their removal from the ovary, but, as a 

 precaution against failure to detect chance fertilization by Cidaris 

 sperm, they were kept for 2 hours before inseminating with the foreign 

 sperm. More than 90 per cent of the eggs so treated fertilized; the 

 typical fertilization membrane appeared shortly after the addition of 

 the sperm. 



The reciprocal crosses were not readily made, and as the time 

 available was short and the problem with which my investigation 

 was concerned was not closely connected with that of specificity 

 in fertilization, no attempt was made to develop a successful technique 

 for these crosses. 



In the cross-fertilized Cidaris eggs, no difference from the charac- 

 teristic normal Cidaris development was noted before the beginning 

 gastrula stage. The fertilization-cleavage interval was not lessened. 

 Cleavage was regular; the micromeres were formed as in other 

 echinoid eggs, and the blastulse had the appearance of normal Cidaris 

 blastulse. In both crosses the mesenchyme cells arose from the 

 sides and around the base of the archenteron, close to the point of 

 union of the archenteron with the wall of the gastrula (plate 3, figs. 

 A to e). In point of time the appearance of the mesenchyme seemed 

 slightly hastened, although not sufficiently to warrant a general 

 conclusion to that effect, as it was within the range of variation 

 between different lots of straight fertilized eggs. 



As to the place of mesenchyme formation, there is no chance for 

 individual variation in the larvae or in different lots of eggs to form the 

 basis of an error in conclusion. In Cidaris gastrulss the archenteron 

 is a straight, slender tube, at whose inner end the migrating mesen- 

 chyme cells may be seen readily (plate 3, fig. j). At the time of the 

 beginning of the formation of mesenchyme the gastrula is exceedingly 

 transparent and the observer may convince himself that there are 

 no formed elements at any place in the blastocoele. Figures 5, c, d, 

 and e, are of optical sections of Cidaris 9 X Lytechinus d^ living 

 gastrulse. In these the primary mesenchyme ceils may be seen 

 clustered at the base of the archenteron and in some cases dispersed 



