504 



DEVELOPMENT OF PRIMITIVE BODY FORM 



gastrula and later embryo to the stage where a continuous antero-posterior 

 coelomic space is formed, comparable to that found in the vertebrates. 



The mesoderm of the late gastrula of Amphioxus is present as a dorso- 

 median band of cells on either side of the notochord, and together with the 

 notochord, occupies the dorsal area or roof of the archenteron as mentioned 

 previously. In the region of the blastopore, the two mesodermal bands diverge 

 ventrally and occupy the inner aspect of the lateral walls of the blastopore 

 (fig. 190F, G; 247B). At about the time of blastoporal closure, the two 

 mesodermal masses of cells, located along the lateral lips of the blastopore, 

 are retracted dorsally, where they come to lie on either side of the notochord 

 (fig. 247C). In this position the two bands of mesoderm and the notochord 

 continue to form the dorsal region or roof of the archenteron until approxi- 

 mately the time when the embryo is composed of 13 to 14 pairs of meso- 

 dermal segments or somites (fig. 249C). (See Hatschek, 1893, pp. '31, 132; 

 Willey, 1894, p. 115; Conklin, '32, p. 106.) When the embryo reaches a 

 stage of development wherein 15 to 16 pairs of somites are present, the 

 notochord and mesoderm have separated entirely from the entoderm (fig. 

 249D). At about this period the neurenteric canal between the metenteron 

 and the neural tube disappears (fig. 249C, D). 



ENTEROCOEU 



MESODERM 



NCHNOCOEL 

 SPLANCHNOCOELS FUSE HORIZONTAL SEPTUM 



Fig. 250. Differentiation of somites in Amphioxus. (A and B from Conklin, '32; C, 

 E, and F after Hatschek, 1888 and 1893; D from MacBride, 1898; all figures are modi- 

 fied.) (A) Somites shortly after separation from entoderm. (B) Later stage, the 

 somites grow ventrally. (C) Somitic wall begins to differentiate into a thickened, dorsal, 

 myotomic area, located near notochord and neural tube, and thinner somatic and visceral 

 areas. (D) Horizontal septum formed which separates dorso-myotomic portion of somite 

 from splanchnocoelic area below. (E, F) Later stages in differentiation of myotome and 

 myocoelic diverticulum. (See text.) 



