40 Hyhridizalion of Echinoids. 



SUMMARY. 

 Part I. 



1. Cidaris tribuJoides is a primitive Echinoid whose early develop- 

 ment is less modified than that of the more modern Echinoids. 



2. In its normal development it is of interest (1) because of the 

 slowness of its development when compared with Lyiechinus and Trip- 

 neustes, (2) in the difference in site of the formation of its mesenchyme, 

 (3) in the place of appearance of the larval skeleton, and (4) in the 

 form of the larva. 



3. The eggs of Cidaris are activated easily by the sperms of 

 Lyiechinus or Tripneustes. These give interordinal crosses. 



4. The larvse obtained from such cross-activations die in the 

 gastrula stage. 



5. Cross-activation with these foreign sperms causes mesenchyme 

 cells to pass into the blastocoele before the beginning of gastrulation, 

 while in the species-fertilized eggs immigration of mesenchyme cells 

 does not occur until after the archenteron has been formed. 



6. It is suggested that the use of the term internal block be limited 

 to block, or inhibition of the conjugation of the germ nuclei, and 

 that the term developmental block be used in designating later inhi- 

 bitions of development. 



7. The use of a modification of the Giinther Hertwig classification 

 of hybrids is suggested. 



Part II. 



1. The eggs of Cidaris, although traUvsparent, are rich in fat and 

 yolk. 



2. A study of species-fertilized eggs shows that half of the eggs 

 contain 37 chromosomes and half 38. Eggs of the group containing 

 37 chromosomes have one V-shaped element, while those of the 

 group containing 38 chromosomes have two V-shaped elements. 



3. Parthenogenetic eggs have 19 chromosomes. 



4. Cidaris sperms are dimorphic, half containing a V-shaped 

 chromosome, half being without this element. Half of the sperma- 

 tozoa contain 19 chromosomes, half contain 18. 



5. Some of the paternal chromosomes lag in division and are 

 eliminated during cleavage in cross-activated eggs. 



6. Investigations on Lyiechinus- Tripneustes and Arbacia-Moira 

 material show alteration in the physical characteristics of the cyto- 

 plasm of the egg by action of the foreign sperm. 



7. The phenomena exhibited suggest that the effects are due to 

 the action of foreign enzymes on the cytoplasmic substrate. 



8. The nucleus in cross-activated Cidaris eggs seems to be supphed 

 with more material than it can utilize. 



