Fourth Stage: The Platodes
Man’s ancestors have substantially the organisation of simple Platodes (at first like the cryptocœlic Platodaria, later like the rhabdocœlic Turbellaria). The leaf-shaped bilateral-symmetrical body has only one gut-opening, and develops the first trace of a nervous centre from the ectoderm in the middle line of the back (Figs. 239, 240).
Fifth Stage: The Vermalia
Man’s ancestors have substantially the organisation of unarticulated Vermalia, at first Gastrotricha (Ichthydina), afterwards Frontonia (Nemertina, Enteropneusta). Four secondary germinal layers develop, two middle layers arising between the limiting layers (cœloma). The dorsal ectoderm forms the vertical plate, acroganglion (Fig. 243).
Sixth Stage: The Prochordonia
Man’s ancestors have substantially the organisation of a simple unarticulated Chordonium (Copelata and Ascidia-larvæ). The unsegmented chorda develops between the dorsal medullary tube and the ventral gut-tube. The simple cœlom-pouches divide by a frontal septum into two on each side; the dorsal pouch (episomite) forms a muscle-plate; the ventral pouch (hyposomite) forms a gonad. Head-gut with gill-clefts.
Seventh Stage: The Acrania
Man’s ancestors are skull-less Vertebrates, like the Amphioxus. The body is a series of metamera, as several of the primitive segments are developed. The head contains in the ventral half the branchial gut, the trunk the hepatic gut. The medullary tube is still simple. No skull, jaws, or limbs.
Eighth Stage: The Cyclostoma
Man’s ancestors are jaw-less Craniotes (like the Myxinoida and Petromyzonta). The number of metamera increases. The fore-end of the medullary tube expands into a vesicle and forms the brain, which soon divides into five cerebral vesicles. In the sides of it appear the three higher sense-organs: nose, eyes, and auditory vesicles. No jaws, limbs, or floating bladder.