Fig. 7 in Precursory siphuncular membranes in the body chamber of Phyllopachyceras and comparisons with other ammonoids

Fig. 7. Successive stages of formation of the septal neck−siphuncular complex in Phyllopachyceras ezoense. A. Stage before anterior migration of the body and development of invagination of the septal epithelium in median dorsoventral (A1) and transverse (A2) sections. B. Development of the invagination of the septal epithelium followed by the gradual migration of the circumsiphonal portion of the body in median dorsoventral (B1) and transverse (B2) sections. Inner and outer layers of the primary conchiolin membranes are secreted by the siphuncular and septal epithelia, respectively. C. Gradual migration of the body and subsequent secretion of the nacreous septum by the septal epithelium. The inner layer of the siphuncular wall is thickened by additional conchiolin membranes secreted by the siphuncular epithelium, and the siphuncular wall at the preceding septal neck region is partly calcified (auxiliary deposit); median dorsoventral (C1) and transverse (C2) sections. Arrows point to the adoral direction.