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Although most types of cytoplasmic organelles accumulate more or less continuously, yolk granules do not appear until oocytes reach a diameter of 35 \u03bcm, and there is stage\u2010specific synthesis of cortical granules in 60\u201080 \u03bcm oocytes. In the nucleus a single nucleolus first appears when an oocyte is 15 \u03bcm in diameter. Then a nucleolus satellite, which is about 3 \u03bcm in diameter, forms in 30 \u03bcm oocytes; this nucleolus satellite later (60\u201070 \u03bcm oocytes) becomes surrounded by 750 nm dense spherical bodies. Large (2\u20104 \u03bcm in diameter) juxtachromosomal spherules occur only in the nuclei of mature oocytes. Microvilli become progressively more numerous and longer until the oocytes reaches a diameter of 90 \u03bcm their tips project 1 \u03bcm beyond the fibrous surface coat, which is 2 \u03bcm thick when well developed. Near the end of oocyte growth, the microvilli retract into the surface coat leaving their pinched\u2010off tips adhering to the outside of the coat.<\/jats:p>","DOI":"10.1002\/jmor.1051470407","type":"journal-article","created":{"date-parts":[[2005,2,24]],"date-time":"2005-02-24T18:58:01Z","timestamp":1109271481000},"page":"475-505","source":"Crossref","is-referenced-by-count":36,"title":["Oocyte differentiation in <i>Urechis caupo<\/i> (echiura): A fine structural study"],"prefix":"10.1002","volume":"147","author":[{"given":"Meredith","family":"Gould\u2010Somero","sequence":"first","affiliation":[]},{"given":"Linda","family":"Holland","sequence":"additional","affiliation":[]}],"member":"311","published-online":{"date-parts":[[2005,2,6]]},"reference":[{"key":"e_1_2_1_2_1","doi-asserted-by":"publisher","DOI":"10.1083\/jcb.35.1.193"},{"key":"e_1_2_1_3_1","doi-asserted-by":"publisher","DOI":"10.1083\/jcb.37.2.514"},{"key":"e_1_2_1_4_1","doi-asserted-by":"publisher","DOI":"10.1007\/BF00968546"},{"key":"e_1_2_1_5_1","volume-title":"Ultrastructure of Fertilization","author":"Austin C. 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