The tube is filled with Xenopus egg cytoplasm plus nuclei reconstituted from sperm chromatin. The nuclei are the green balls. When the extract goes into mitosis, the nuclei disappear; when it exits mitosis, the nuclei reform.
Notice that after the first mitosis, the subsequent mitoses sweep up and down the tube in waves. This is due to trigger waves of Cdk1 activation.
Adapted from Chang JB, Ferrell JE Jr. Mitotic trigger waves and the spatial coordination of the Xenopus cell cycle. Nature. 2013 Aug 29;500(7464):603-7
