© 1977 by The Society for Integrative and Comparative Biology
Embryology of Phoronida
Station Marine d'Endoume, Rue de la Batterie-des-Lions 13007 Marseille, France
Fertilization in the Phoronida appears to be internal. Three different types of eggs were found: (1) Eggs rich in yolk, about 125µ in diameter, which are retained in the parent's tube, without a true pelagic life; (2) Eggs moderately rich in yolk, about 100 µ in diameter, brooded up to the actinotroch stage in the lophophoral concavity owing to the nidamental glands, with a more or less long pelagic life; (3) Eggs, yolk-poor, about 60 µ. in diameter, which are directly discharged into sea-water and which have a long pelagic life.
Cleavage in the Phoronida is total, equal or subequal. The pattern is typically radial though biradial in some stages, but there are instances in which the blastomeres exhibit a spiral appearance.
The gastrula arises generally by emboly. The blastocoel is extensive in embryos of type 3 (see above) and virtually obliterated by wall compression in type 2. The blastopore is reduced to an anterior remnant.
The differentiation of the ectoderm leads to the formation of the preoral lobe, the apical plate, the tentacular ridge, the nephridial anlage, the oesophagus (issued from the posterior part of the vestibule) and the mouth which does not originate as a stomodeum; the blastopore is located between oesophagus and stomach.
Differentiation of the archenteron (endoderm) produces the stomach, the intestine and the anus which opens by perforation of the ectoderm, without formation of a proctodeum. The anus appears to be independent of the blastopore.
The mesodern originates as isolated cells proliferated from the anterior and ventrolateral areas of the archenteron, in two phases. The mesoderm is formed in a modified enterocoelous manner. The protocoel is produced first from the anterior archenteric wall and occupies the cavity of the preoral lobe; the metacoel originates from the ventrolateral mesodermal proliferations. The mode of formation of these cavities seems to vary with species.