DEVELOPMENT OP PARA VORTEX GEMELLIPARA 475 



Since the term 'yolk-nucleus' has been applied by different 

 authors to several different cell structures in many groups of 

 animals, it seems advisable to pause here, and briefly to de- 

 scribe these various types. The name 'Dotterkern' was first 

 given by Carus in 1850 to a body which had been described 

 by von Wittich as lying in the cytoplasm of immature spider 

 eggs (Lycosa, Tegenaria and Thomisus). Since then observers 

 have reported the presence of a yolk-nucleus in the eggs of echino- 

 derms, molluscs, worms (Trematodes), crustaceans, myriapods, 

 insects, fishes, amphibians, birds and mammals, including man. 



As explained in Korschelt and Heider's Textbook of Inverte- 

 brate Embryology, the bodies described for the various animals 

 as yolk-nuclei may be classified in three groups, (a) true 'Dotter- 

 kerne' which are supposedly concerned with the deposition 

 of yolk in the egg, (6) bodies more or less similar to yolk-nuclei 

 in appearance, and (c) structures which are to be identified 

 as attraction spheres. 



Calkins in his preliminary notice published in 1895 on the 

 origin and changes of the yolk-nucleus in the eggs of the earth- 

 worm, Lumbricus terrestris, points out that ''the work of various 

 observers may be divided into two classes according as the j^olk- 

 nucleus is conceived to be of cytoplasmic or nuclear origin." 



As examples of the first class Calkins cited, among others, 

 the work of Lubbock ('61) who regarded the yolk-nucleus as 

 a thickening of the plasm forming the vitellus, of Sabatier('83) 

 who asserted that in the spider the yolk-nucleus arises in the 

 cytoplasm near the nucleus and wanders to the periphery where 

 it degenerates; of Hall ('90) whose observations on the chick 

 led him to the conclusion that the yolk-nucleus has its origin 

 in a mass of granules near the germinal vesicle to which it 

 may send out prolongations; and the work of Monticelli ('92) 

 who claimed that in the Trematodes there is no connection 

 between germinal vesicle and yolk -nucleus, but that the latter 

 arises as a protoplasmic differentiation of the cytoplasm. Jor- 

 dan ('93) came to a similar conclusion in regard to the yolk- 

 nucleus of the newt. 



