282 KANSAS UNIVERSITY SCIENCE BULLETIN. 



II. HISTORICAL STATEMENT. 



There has been much confusion regarding the term "yolk 

 nucleus." It has been applied to so many formations in the 

 cytoplasm that its present meaning is very indefinite. Not 

 only have we the one word applied to several different struc- 

 tures, but we are further embarrassed by having several differ- 

 ent names for the same structure. Thus yolk-nucleus, corps 

 vitellin, or vitelline body, and dotterkern are all applied to one 

 structure by some writers, while sharp distinctions are made 

 by others. The tendency now is to separate the terms, and call 

 the peripheral globules, which collect in the early eggs just 

 about the time of the yolk formation, yolk nuclei. Vitelline 

 body is applied to the single large formation near the nucleus, 

 arising also about the time of the collection of yolk. 



The diffused yolk nuclei are of two kinds. Small bodies oc- 

 curring very near the egg membrane, and staining densely, 

 have been described by both Foot (4) and Calkins (2) in the earth- 

 worm. These occur in the egg of Limulus, as shown by Mun- 

 son (6). The other diffused bodies lying in the cytoplasm, to 

 which the term yolk nuclei has also been applied, are much 

 larger than the peripheral formations, and lie scattered irregu- 

 larly between the nucleus and the cell-wall. Calkins (2) in his 

 work on Lumbricus, shows these bodies first collected in one 

 mass around the nucleus, and later break up into separate 

 spheres. They are homogeneous, and finally disintegrate, aid- 

 ing in someway the yolk formation. Crampton (3) describes in 

 Molgula a similar collection of material around the nucleus, 

 and a subsequent breaking-up and scattering of the mass, 

 though in this case the bodies are not so large or so definite as in 

 Lumbricus ; here the deutoplasm mingles more uniformly with the 

 cytoplasm. We see, therefore, that while members of this class 

 of yolk nuclei correspond to the peripheral ones in being many in 

 number and in being scattered, still their origin is apparently 

 different. The peripheral bodies certainly seem to arise in the 

 cytoplasm ; at least, they are not a direct product of the nucleus. 

 The others appear first close against the nucleus, and, accord- 

 ing to Calkins, are formed from a portion of the chromatin 

 reticulum cast out by the nucleus. 



