IOO CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



REPRODUCTION 



In the class Echinoidea there are no species which are capable 

 of asexual reproduction, such as occurs in some of the Asteroidea, 

 the production of eggs which require fertilization being the only 

 means of reproduction in this class. 



The echinoids are of separate sexes, although it is impossible 

 to distinguish the males from the females externally. Each 

 female discharges vast numbers of eggs into the water during the 

 breeding season of each year of her life. In one of the foreign 

 species a large female may produce twenty million eggs yearly. 

 Owing to the gregarious habits of the animals, the eggs stand a 

 reasonable chance of being fertilized by the sperm cells which 

 are discharged in enormous numbers by the male urchins at the 

 same season. 



It often happens, as is also the case with the starfish, that 

 certain animals fail to mature and discharge their sexual products 

 at the normal season, but eggs discharged at other periods of the 

 year must almost always fail of fertilization or development. 



The eggs are so numerous and easily obtained that they have 

 become classical objects for the study of the processes of fertili- 

 zation and early development. The suggestions on page 50 re- 

 garding the study of starfish eggs will be found equally appro- 

 priate for those of the urchins. 



DEVELOPMENT 



As illustrated by Fig. 15, the immature eggs in the ovary of 

 the urchin are each provided with a large germinal vesicle. 

 After the two polar bodies are formed, the egg is discharged into 

 the water; it then settles slowly to the bottom and is ready for 

 fertilization. A single sperm cell, which may be swimming about 

 in the water in the vicinity, may enter the egg. After such union 

 the fertilized egg divides regularly into 2, 4, 8, and 16 cells, 

 called blastomeres, as illustrated in Fig. 15. 



These divisions take place very rapidly, the first cleavage 

 occurring in about an hour and a quarter after fertilization, and 

 succeeding cleavages at intervals of an hour or less. 



A hollow sphere of cells known as the blastula is then formed. 

 In about twelve hours after fertilization the blastula becomes 



