From the evolutionary point of view the coelom-pouches are, in any case, older than the chorda; since they also develop in the same way as in the chordonia in a number of invertebrates which have no chorda (for instance, Sagitta, Figures 1.76 to 1.78). Moreover, in the amphioxus the first outline of the chorda appears later than that of the coelom-sacs. Hence we must, according to the biogenetic law, postulate a special intermediate form between the gastrula and the chordula, which we will call coelomula, an unarticulated, worm-like body with primitive gut, primitive mouth, and a double body-cavity, but no chorda. This embryonic form, the bilateral coelomula (Figure 1.81), may in turn be regarded as the ontogenetic reproduction (maintained by heredity) of an ancient ancestral form of the coelomaria, the Coelomaea (cf. Chapter 2.20). In Sagitta and other worm-like animals the two coelom-pouches (presumably gonads or sex-glands) are separated by a complete median partition, the dorsal and ventral mesentery (Figure 1.78 dm and vm); but in the vertebrates only the upper part of this vertical partition is maintained, and forms the dorsal mesentery. This mesentery afterwards takes the form of a thin membrane, which fastens the visceral tube to the chorda (or the vertebral column). At the under side of the visceral tube the coelom-sacs blend together, their inner or median walls breaking down and disappearing. The body-cavity then forms a single simple hollow, in which the gut is quite free, or only attached to the dorsal wall by means of the mesentery. The development of the body-cavity and the formation of the chordula in the higher vertebrates is, like that of the gastrula, chiefly modified by the pressure of the food-yelk on the embryonic structures, which forces its hinder part into a discoid expansion. These cenogenetic modifications seem to be so great that until twenty years ago these important processes were totally misunderstood. It was generally believed that the body-cavity in man and the higher vertebrates was due to the division of a simple middle layer, and that the latter arose by cleavage from one or both of the primary germinal layers. The truth was brought to light at last by the comparative embryological research of the Hertwigs. They showed in their Coelom Theory (1881) that all vertebrates are true enterocoela, and that in every case a pair of coelom-pouches are developed from the primitive gut by folding. The cenogenetic chordula-forms of the craniotes must therefore be derived from the palingenetic embryology of the amphioxus in the same way as I had previously proved for their gastrula-forms.