Plate 3

Fig. 1 Cross section through a branch tip. Note the columnar ectoderm enclosing skeletal elements and endoderm. The regular arrangement of coenosarc tubes, and therefore of skeletal elements, is clearly seen.

Fig. 2 An enlarged portion of Fig. 1. Secretory ectodermal cells are seen at the junction of 3 ribs of skeleton.

Fig. 3 Cross section of a branch tip 10μ from Fig. 1.

Fig. 4 The area of Fig. 3 corresponding to the area shown in Fig. 2. The skeletal structures are reduced, but in their place are secretory ectoderm cells which show a granular cytoplasm.

ect, ectoderm; end, endoderm; p, polyp base; sec ect, secretory ectoderm; sk, skeleton.