The Behaviour of the Nucleolus during Oogenesis. 123 



rodlets which broke up into smaller granules which were converted 

 into fat. Nucleolar extrusion in Myxine has been observed in 

 pigment cells, connective tissue cells, blood cells, many gland cells, 

 and in the male germ cells. 



Schreiner regards the rodlets formed by the extruded nucleolar 

 fragments as mitochondria, and considers this to be a case of 

 conversion of mitochondria into fat. The transformation of mito- 

 chondria into fat globules has been described by Murray in the cells 

 of a transplantable sarcoma of Cavia {13), where the mitochondria 

 at first, either moniliform or filamentous, break up to form granules 

 which become swollen and eventually are converted into fat 

 globules. No mention is made as to the nucleolus being concerned 

 in this process, but nucleolar extrusions are described and figured 

 in cells of the same tumour, their ultimate fate being uncertain. 



Nucleolar extrusions in somatic cells seem to be specially 

 related to active metabolism of the protoplasm. Instances of this 

 are the considerable nucleolar extrusions which occur in the 

 epithelial cells of the silk glands of insects during the formation 

 of silk, and in the cells of squamous epithelium of Mammals 

 during the formation of keratin. 



It is doubtful to what extent the nucleoli of oocytes are 

 comparable with those of somatic cells. Usually there is a single 

 nucleolus present in the young oocyte. This is the case in most 

 Echinoderms, many Coelenterates, worms, molluscs and rodents (2). 

 After the oocyte has undergone the prophases of the heterotypic 

 division, the chromosomes as such generally seem to disappear {5). 

 This seems to be due to their spreading out in order to exert their 

 maximum influence in the growing cell. Confirmation of this is 

 afforded by the work of Marechal on the oocytes of Elasmobranchs 

 {13), where after the diplotene stage the chromosomes spread out 

 irregularly to form a structure which has been compared to a 

 cylindrical chimney brush. 



During the growth stages when the chromosomes are becoming 

 less distinct in the nucleus of the young oocyte, the nucleolus 

 makes it appearance, and grows considerably in size. Usually 

 there is one large plasmosome at the beginning. During the early 

 stages of oogenesis in Periplaneta and Libellula {8), Hogben has 

 recently shown that there is an extrusion of oxyphil material from 

 such a nucleolus, and he concludes that this is connected with 

 yolk formation and that "the chromatic organization of the nucleus 

 retains its integral continuity throughout this phase." This is 

 contrary to the opinion of Schaxel {14), who working on the 

 oocytes of Echinoderms and Tunicates came to the conclusion that 

 chromatin was emitted from the nucleus and played an important 

 part in yolk formation, a conclusion which is open to considerable 

 doubt in the light of recent work. 



In the oocyte of Saccocirrus, Gatenby {6) has found a 



K 2 



