The superficial cleavage is a type of meroblastic (partial) cleavage yolk-rich eggs . It is found in many arthropods and is especially characteristic of insects . It was examined more closely in Drosophila melanogaster .
From the cell nucleus of the zygote , many more cell nuclei arise through many synchronous and rapidly successive nuclear divisions without subsequent cell divisions , resulting in a multinucleated syncytium . Most of the nuclei then migrate to the periphery of the syncytium, and the plasma membrane turns inside out between neighboring nuclei (superficial, i.e. superficial furrowing), so that each nucleus lies in a honeycomb that is open towards the inner yolk. This stage is known as syncytial blastoderm . Finally, each honeycomb is separated from the yolk as a closed cell. This cellular blastoderm stage is reached in D. melanogaster after about two and a half hours, and the blastoderm then consists of about 6,000 cells. Then gastrulation begins .
The other type of meroblastic furrow is the discoidal furrow , in which the nucleus of the zygote lies on the surface and from this animal pole a spreading germinal disc is created by cell division (furrow) . This type of groove is u. a. in birds and reptiles .
Small eggs that contain little yolk, such as those of humans and mammals , have a holoblastic or total furrow. The yolk-poor eggs of many originally aquatic animals such as sea urchins are also holoblastic . While mammalian eggs only need a small amount of yolk because the developing embryo is nourished by the maternal organism very early on, sea urchins form very small larvae that can feed on their own.