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In discoidal cleavage, the cleavage furrows do not penetrate the yolk. The embryo forms a disc of cells, called a blasto-disc, on top of the yolk. Discoidal cleavage is commonly found in monotremes , birds , reptiles , and fish that have telolecithal egg cells (egg cells with the yolk concentrated at one end).
Superficial (insects [9] [17]) The end of cleavage is known as midblastula transition and coincides with the onset of zygotic transcription . In amniotes, the cells of the morula are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast , which does not contribute to the formation of the ...
During cytokinesis, centrolecithal zygotes undergo meroblastic cleavage, where the cleavage plane extends only to the accumulated yolk and is superficial. This is due to the large dense yolk found within centrolecithal eggs and triggers a delayed embryonic development.
Cleavage itself is the first stage in blastulation, the process of forming the blastocyst. Cells differentiate into an outer layer of cells called the trophoblast, and an inner cell mass. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable. They are still enclosed within the zona pellucida. This ...
It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). [1] It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma meaning "skin". [2]
Sulcus" is a Latin word that means a furrow or groove, commonly used to mean a fold, fissure or furrow of the brain (example: lateral sulcus). [13] [14] In popular usage the area is commonly referred to as a cleavage of breasts. In surgical parlance, the cleavage or intermammary cleft is also known as the "medial definition" or "medial fold" of ...
Blake Lively is the queen of going topless underneath a blazer.. Lively, 36, wowed in an embroidered set while posing for an It Ends With Us photocall in London on Thursday, August 8. Her outfit ...
1. Bilateral cleavage (cephalopod molluscs) 2. Discoidal cleavage (some fish [the hagfishes, chondrichthyans and most teleosts], sauropsids [reptiles and birds], monotremes) B. Centrolecithal (yolk in center of egg) Superficial cleavage (most insects)