YOLK FORMATION 145 



closely-related Mollusca, Helix, Limncea and Patella. In the 

 first two animals, the yolk is either derived from the cyto- 

 ])lasm, or else by a metamorphosis of the Golgi elements which 

 are diffuse. In Patella the Golgi dictyosomes are found attached 

 to the yolk spheres, and there does not seem much doubt that they 

 are closely connected. 



In amphibia and insects a distribution of the mitochondria to 

 the periphery, where the yolk always appears, suggests a possible 

 connection ; but in the frog, where the mitochondria stain 

 differentially to the yolk, no transitional forms have been found. 

 Certainly in insects the yolk and the Golgi apparatus appear to be 

 unconnected. The yolk here is possibly derixed from the nucleoli 

 (Hogden, vide infra). 



In Ascidia the work of Hirschler (49) shows that yolk arises by 

 growth of the mitochondria and subsequent fusion with the Golgi. 

 (This is denied by Harvey, vide infra.) In embryonic cells the 

 swollen mitochondria (i.e., yolk) shrink back to their original size. 



In Ascaris, yolk is derived by the swelling of the mitochondria. 

 No contact with the Golgi apparatus is seen. Fat appears in the 

 cytoplasm distinct from the rest of the inclusions. 



The relationship between mitochondria and fat is gone into by 

 Gatenby and Woodger, but there is little definite evidence except 

 that the mitochondria can swell up to form fat (Schreiner, Murray 

 and Dubreuil). 



Gatenby explores the possibility that the archoplasm (= idio- 

 plasm) associated with the Golgi elements may become loaded 

 with leipins and fats under the action of the Golgi, and thus 

 change into yolk (vide, e.g., Patella, supra). 



Apart from these well-known inclusions, the relationship of the 

 so-called yolk nuclei and chromidia required much further investi- 

 gation, as the definitions given them show that they are not always 

 homologous structures. Furthermore, the part played by the 

 nucleolus, illustrated by the case of Saccocirrus (Gatenby), and 

 insects (Hogben), was not correlated with the other cell activities. 



It will be seen from this that practically all the known cell 

 inclusions, including even the chromatin of the nucleus (crude 

 technique had attributed a chromatin origin to many chromidia 



