THE ONTOGENESIS OF VERTEBRATES 61 



trula assumes the form and position shown in Fig. 13, b, in 

 which it becomes placed horizontally, with the apical pole for- 



a 



ZtHHHHSSS 



ant, 



FIG. 13. Diagrams of gastrulae. [Based on models of Amphioxus by 

 HATSCHEK.] 



(a) Typical gastrula, as in Fig. 12, VIII, but differently placed, for comparison 

 with the others. (b) Early gastrula of Amphioxus, a probable ancestor of the 

 vertebrates, (c) Later embryo of the same. 



xy, primary axis, t. e., that of the gastrula; ab, secondary axis, that of the adult 

 Amphioxus; en, neural canal; cne, neurenteric canal; np, neuropore; ntc, notochord. 



ward and the protostome posterior and dorsal and in the 

 median line. The plan of structure is a bilateral one, with 



