Desciiptiou uf tlio Modiücations of certain Organs. 639 



5. The larynx and the manubrium stcrni of Howling Monkeys. 



Albuecht») has drawii attention to the cleft manubrium of the 

 Howüng Monkeys as „the first positive proof of Darwin's theory of 

 adaptation and inheritance". 



He sums up the results of his investigations as follows. 



There are Monkeys, belongiug tq the Genus Mycetes, 1) which 

 possess au unpaired manubrium sterni like all other Mammals; 2) in 

 ^Yhich the premanubrium is cleft whilst the postmanubrium (equal to 

 the copulae between the first and second sternal ribs) is still unpaired 

 or united; 3) in which the whole manubrium is cleft into a right 

 and a left half. The second case is by far more frequent than the 

 others, and now represents the normal or actual conditiou. Since 

 obviously the Howling Monkeys are descendants of Monkeys without 

 a cleft manubrium, the first case is, concerning Mycetes, atavistic, 

 the third case (complete fissure) „epigonistic". 



According to Albrecht the fissure can be explaiued only in this 

 way, that the two halves of the manubrium have been prevented from 

 joining each other in the middle line by the colossal development of 

 the basihyal and the thyreoid cartilages, which is known to occur 

 already in the 7'*" week of embryonic life. Agaiu, such an enlargement 

 of the howling apparatus cannot have been acquired by the embryos, 

 but must have been transferred to them by their ancestors, which deve- 

 loped it through excessive use. The fissure, originally teratological, has 

 become normal, and there will probably come the time, when all the 

 species of Mycetes will possess a complete fissura or „sternoschisis 

 manubrii". The enormous enlargement of the basihyal bone and of 

 the thyreoid cartilage occurs in both sexes, although in the female to 

 a smaller extent. So far Albrecht. 



But there exists a difficulty concerning the relative position of 

 the laryngeal apparatus and the manubrium sterni. In the adult the 

 thyreoid cartilage lies from 1 to 2 cm in front of or headwards 

 from the manubrium, and not inside of, or between , the two 

 halves of the manubrium and the first sternal ribs. The mere size 

 of the larynx cannot therefore interfere with the framework of 

 the ehest, at least not when the animal is at rest. But when 

 the Monkey, whilst howling, inflates the laryngeal sacs and distends 



1) P. Albeecht, Sur les elements morphologiques du manubrium. 

 sterni, Bruxelles 1884. 



