182 III. THE EVOLUTION OF VIRUSES 



the virus of Nairobi sheep disease is said to be transmitted through 

 the eggs of a brown tick, the vector (98). 



Usually seed from infected plant gives healthy progeny, but a 

 few plant viruses are believed to invade and survive in the seeds set 

 by infected plants, thus making possible the transmission of the 

 virus through seed, a phenomenon analogous to the virus transmission 

 through the eggs laid by infective arthropods. Seed transmission is 

 said to be more characteristic of viruses of leguminous plants than 

 any others ; a number of those have been described as seed-borne. 



However, not all the seeds set by diseased plants are infected, 

 and the proportion of infected seeds to virus-free ones depends on 

 the length of time the parent plant has been infected (99). This may ' 

 support the customary idea that the virus is mechanically carried by 

 the seed. But in the opinion of the writer this should be regarded as 

 analogous to the above cited fact that the ability of insects to cause 

 infection depends upon the initial charge of the plant viruses, and 

 also to the fact that both the duration of antibody production and the 

 antibody amount produced tend to be directly proportional to the anti- 

 gen amount administered. 



Such transmissibility of virus to offspring through egg or seed 

 involves, to the belief of the writer, the utmost important significance. 



As will be discussed in great detail in Part V, there are many 

 good reasons to conclude that in germ cells the replicas given rise to 

 by viruses are kept in an altered form. The virus in itself, accor- 

 dingly, may unable to be demonstrated as such. The altered replica, 

 however, may gradually recover its original form as the germ cell 

 develops to an adult organism, so that the virus may be revealed at a 

 stage of the development. 



Usually young organisms having emerged from the eggs of the 

 infected insects are not infective immediately after hatching, but will 

 become so only after variable waiting periods. For example, according 

 to Black (97) no insect raised from eggs laid by an insect vector 

 infected by clover-club leaf virus can be infectious until three weeks 

 after hatching, and most infections are obtained when the insects are 

 between 6 to 12 weeks old. Such a failure of newly hatched insects 

 in inducing infection may not be impossible to be attributed to too 

 small virus amounts contained in the egg to cause the infection; if 

 so, the waiting period may be the time required for the multiplication 

 of this virus. The writer, however, regards this fact as an example 

 of the reversion of the replica in the developing insect. It should be 

 noted that plants generated from the seed inheriting a virus like- 

 wise never reveal the disease symptoms until the plants grow up to 

 a certain extent. 



