291 



It has been shown that a balancer-forming region is present as 

 early as the 3 mm stage. Possibly a balancer- forming substance is 

 present in the unsegmented egg, but my results throw no light on 

 this latter point. 



It is hoped that the above results will form some contribution 

 to the science of correlative embryology. In the development of any 

 organ such as the eye or the balancer, composed of tissues from 

 distinctly diiferent sources, one may expect to find correlations. It 

 is probable that one of the tissues is more specialized than the other 

 and directs the development of the organ. It is of course necessary 

 to show clearly in as many cases as possible where such correlations 

 exist before any successful attempt can be made to explain how they 

 are effected. The work of Spemann, Lewis, and others on the lens 

 has attracted widespread interest, and there are other organs where 

 correlations of almost equal interest may be shown to exist. 



Nachdruck verboten. 



The Histology of the Yolk Sac of a 9,3 mm Human Embryo. 



By H. E. Jordan, A. M. 



(From the Pathological Laboratory of Cornell University Medical 

 College, New York.) 



With 8 figures. 



The object of this paper is to report the results of a micro- 

 scopic study of the yolk sac of an early human embryo. As far as 

 I have been able to discover, the first detailed description of the 

 structures of the human yolk sac was pubhshed by Graf Spee in 1896; 

 another was published by Paladino in 1901, and a third by Meyer in 

 1904. In the "Entwickelungsgeschichte der Menschen-Affen", Selenka 

 has published some observations on the yolk sacs of apes. 



Graf Spee's work is a comparative study of the early stages of 

 the human yolk sac. He ascribes to this organ a glandular structure 

 and function and says that he finds no mention of a glandular tissue 

 in the human yolk sac by earlier authors. He reports a striking simi- 

 larity both as regards microscopic structure and function between the 

 yolk sac and the liver, and states his belief that the former inaugu- 

 rates metabolic processes for the animal economy which are sub- 

 sequently assumed by the latter. 



Selenka (1899) says that in Gibbon "the yolk sac shows in its 

 early stages a glandular structure" ; that in ruminants, apes, and man 



19* 



