6 



Hybridization of Echinoids. 



to chance fertilization with Cidaris sperm, they were kept for two 

 hours before making the inseminations. A portion of the eggs was 

 always kept as an unfertilized control. These statements regarding 

 the ease of cross-fertilizing apply to conditions existing from March 

 5 through March 18, 1912, the only period in which I have had the 

 opportunity of working on these forms. 



In its normal development Cidaris proved of interest: (1) because 

 of its slowness of development when compared with Lytechinus and 

 Tripneustes; (2) in the difference in site of its mesenchyme formation ; 

 (3) in the place of appearance of the larval skeleton; (4) in the form 

 of the larva. 



Rate of Development of CmARis compared with that of Other 



Echinoids. 



Following fertilization, the various phases of the nucleus during 

 division may be followed readily. The beginning of the anaphase 

 of the first division is reached about 50 minutes after insemination. 

 The cleavage is like that of the eggs of other echinoids, the formation 

 of the micromeres being as in Lytechinus and Tripneustes. The 

 blastula stage is reached in 16 to 18 hours, the eggs from a given 

 female developing uniformly. The variation in rate of development 

 of different lots of eggs is not altogether determined by temperature, 

 since mixed lots of eggs, although nearly all may develop normally, 

 do not develop at a uniform rate. Gastrulation begins in 20 to 23 

 hours; mesenchyme formation begins in 23 to 26 hours, the mesen- 

 chyme cells arising from the inner end of the archenteron; chromat- 

 ophores appear in about 44 hours; the enterocoele arises as a single 

 pouch in 44 to 50 hours; in 55 hours two enterocceles may be seen, 

 formed by the division of the single vesicle; in 72 to 73 hours the first 

 skeletal spicules may be noted. 



Even at this time, the beginning of the fourth day, the body has 

 not begun to assume the form of an echinopluteus, and it is not until 

 the fifth day that the arms begin to push out. These facts are of 

 interest when compared with those of the development of Lytechinus. 

 Here the biastulse reach a swimming stage in 53^2 hours after the 

 insemination of the egg; mesenchyme cells begin to push into the 



