Involution - Internalization of Mesoderm
As by now mentioned, in the frog Xenopus (and probably in other amphibian species as well) the cells of presumptive mesoderm are in the deeper layers of the dorsal and lateral marginal zones beneath the superficial layer that is presumptive endodermal. The complete mesoderm is internalized through involution over the blastoporal lips. , at the same time with the involution of endoderm of the superficial layer the chorda- mesoderm situated in the dorsal marginal zone (gray crescent) involutes over the dorsal lip, the presumptive somitic mesoderm over the lateral lips and the presumptive lateral plate mesoderm over the ventral lip. As the dorsal lip is formed first and venual lip last the involution first starts in the region of the dorsal marginal zone followed successively in the lateral and ventral regions of the lateral marginal zone. The mesodermal cells of deep layers first interdigitate creating a single thick layer in which cell division as well occurs. The layer together with the overlying superficial endodermal layer converges in the direction of the blastoporal lips, where the edges of the two layers break up into separate cells, which now, involute individually and move into the interior.
The involuting mesodermal cells retain their deep position throughout and after involution. The cells of the presumptive chorda-mesoderm involute over the dorsal lip in successive waves and one time inside they migrate actively as streams anteriorly towards the animal pole. The path of migration is beside the mid-dorsal line between the non-involuted layer of the surface of animal zone above and the layer of involuted endodermal cells below. The cells of first such types of streams reach the future head region where they make the head mesoderm. They are followed by other successive streams of involuted cell that make the notochord above the archenteron. Internalization of the presumptive somitic and lateral plate mesoderm of the lateral marginal zone occurs in the same way. After involution over the lateral and ventral blastoporal lips correspondingly, these cells move in an anterior direction to form a loosely organized layer called mesodermal mantle on either side of archenteron.