5o8 LIFE : OUTLINES OF GENERAL BIOLOGY 



remarkable fact in regard to Bonellia. Out of the eggs there emerge 

 minute free-swimming larvae, which are at first all alike. After a 

 short time some of them sink on to the substratum, and these 

 develop into the large females. But others, which settle down on 

 the proboscis of an adult female, begin to absorb from the skin, 

 and suffer an arrest of development. They develop into the pigmy 

 males. 



Using an ingenious device of staining the cells of the proboscis 

 without killing them, Baltzer was able to prove that the young 

 males absorbed nutritive material from the skin of the adult female. 

 Sometimes he shook off the young males before they had been 

 sedentary for more than a short time on the proboscis of the female, 

 and these developed into strange inter-sex forms, intermediate 

 between males and females. In short, it was proved conclusively 

 that the larvae which settle down in freedom develop into females, 

 whereas those that settle down in parasitism develop into males of 

 microscopic dimensions and with most of the organs in an arrested 

 state. 



At a certain stage the minute young males leave the proboscis 

 and go into the mouth of their bearer. There they undergo a final 

 change. They then emerge and enter the reproductive duct, where 

 they remain for the rest of their life, fertilising the eggs before these 

 are set free. 



But Baltzer has been able to take another step of much interest. 

 He has made an extract of the proboscis and finds that it has a 

 poisoning effect on small animals such as Slipper Animalcules, 

 threadworms, small crustaceans, and tadpoles. In very weak solu- 

 tions it exerts a marked deteriorative effect on the young pigmy 

 males, and a concentration of one part in three thousand is rapidly 

 fatal. An extract of the wall of the female's reproductive duct, in 

 which the parasitic male takes up its abode, is not injurious, there- 

 fore it seems practically certain that the arrest of development 

 experienced by the larvae on the proboscis is due to the absorption 

 of some specific substance from the female's skin. The only step 

 awanting is to treat the free-swimming larvae with very weak 

 solution of the extract. It should make them all males! 



To sum up: Baltzer concluded that the minute free-swimming 

 larvae of Bonellia are at first all alike; that some settle down on the 

 substratum and develop into large females; that others settle on 

 the proboscis of a full-grown female and, absorbing a secretion of 

 the superficial cells, suffer arrest of development, and become pigmy 

 males. If the sojourn on the proboscis of the adult is artificially 

 shortened by shaking the larvae off, they develop into inter-sex 

 forms. It was shown experimentally that an extract of the proboscis 

 skin has a deteriorative effect on various small organisms. All this 

 is strongly suggestive of the conclusion that the primary difference 



