The cytological observation of fertilization in clam Cyclina sinensis

The fertilization of clam Cyclina sinensis and the first cleavage were histologically and cytologically studied via paraffin section with haematoxylin-eosin staining under light microscope.The mature eggs of the clam were found to receive the sperm penetration(i.e.fertilization) during metaphase of first meiosis.The sperm penetration was observed 6 minutes after the eggs and sperm were mixed at water temperature of 23.2 ℃.The first polar body was found 15 minutes and the second polar body 21 minutes after the eggs and sperm were met at 23.2 ℃.The male pronucleus was formed before releasing of the second polar body,and then the female pronucleus occurred quickly,the female and male pronuclei becoming together in the ovum center.The zygotes from the female and male nuclei were formed 42 minutes after the eggs and sperm were mixed,and the first cleavage(two celled stage) was observed about 50 minutes after fertilization.