THE DEVELOPMENT OF AMPHIOXUS 29 



extending from each side completely across the entire chorda. 

 By the time nine or ten somites are formed the notochord 

 becomes completely cut off from the endoderm layer, and the 

 two margins of the endoderm which were originally along the 

 outer margins of the mesoderm folds, come together in the 

 mid-line beneath the notochord, enclosing what remains of 

 the archenteron, as the definitive gut cavity or enteron (mesen- 

 teron) (Fig. 8, E). Finally vacuoles appear in the notochord 

 and, increasing in size, lead to the obliteration of the cellular 

 structure. The nuclei are displaced dorsally and ventrally 

 and a typical "notochordal tissue" is formed before the close 

 of the embryonic period (Fig. 9, C). 



3. The Mesodermal Somites and Ccelom 



After hatching, mesodermal somites posterior to the two then 

 present continue to form successively in the elongating meso- 

 derm folds. The cavities of the first two pairs do not become 

 completely separated from the archenteron until six to eight 

 pairs have been formed and constricted off, and their enteroccels 

 remain as definite spaces throughout their development. In 

 the somites just posterior to the first two pairs the enteroccels 

 close for a time just after the somites are formed, and in the 

 more posterior ones, even before the mesoderm is quite cut off 

 from the endoderm. In these somites cavities reappear later, 

 and the enterocoels may be said to be virtually if not actually 

 present throughout, for the cells composing their walls remain 

 definitely arranged about a central point in the regions of the 

 enteroccels (Fig. 10). Still farther posteriorly the mesoderm 

 folds develop as solid masses without any cavities whatever at 

 any time during their formation, but the condition in the ante- 

 rior region demonstrates that Amphioxus is a true enteroccelo- 

 mate organism. By the time fourteen pairs of somites have 

 been formed the mesoderm folds cut off completely from the 

 archenteric wall, and the remainder of the mesoderm is formed 

 directly from the proliferating cell region around the neurenteric 

 canal (blastopore), without ever having been included as a part 



