776 RESPIRATION AND HEAT-PRODUCTION [pt. iii 



4-22. Light-production in Embryonic Life 



Heat is, of course, not the only form of energy which animals can 

 radiate. Luminous organisms occasionally have luminous eggs as 

 the following table shows : 



The beetle's eggs in question shine before fertilisation, while still in 

 the ovary. It would be interesting to know what component is 

 missing from the early developmental stages of Beroe. In the case of 

 Pyrosoma, according to McKinnon, the luminescence is due to the 

 presence of symbiotic bacteria which are transmitted to each egg as 

 it is laid by a special mechanism in the parent and are afterwards 

 distributed among the blastomeres during the cleavage stages. 

 Similar events take place in certain cephalopods which produce 

 luminous eggs. 



McKinnon also points out that other symbiotic organisms may be 

 transferred to eggs. Thus the reinfection of the termites such as 

 Hylecoetus dermestoides with fungal spores is brought about by the 

 smearing of the spores on to the egg as it passes down the oviduct and 

 the subsequent consumption by the larva of its own egg-case after 

 hatching. The wood-wasp Sirex and the olive-fly Dacus are other 

 instances of this. Moreover, the transmission of the symbiotic cel- 

 lulose-fermenting yeasts from generation to generation occurs in a 

 similar way in various beetles (the death-watch beetle, Xestobium 

 rufovillosum (Staniland); Sitodrepa panicea (Breitsprecher)), also in a 

 large number of homoptera (Richter) and in the rice-weevil, Calandra 

 oryzae (Mansour). The whole subject has been well reviewed by 

 Buchner. 



