26 OUTLINES OF CHORDATE DEVELOPMENT 



somites are at first nearly spherical or cubical, box-like divi- 

 sions, each containing a portion of the original archenteric 

 space now to be known as the enterocoelic cavity of the somite 

 or enteroccel, which in these early stages remains in direct 

 though narrowed continuity with the archenteron; later this 

 connection becomes entirely lost (Fig. 8, D). This transverse 

 cutting up of the continuous mesoderm folds and their cavities 

 into somites and enteroccels proceeds, in somewhat modified 

 form, from the anterior region posteriorly, as the embryo 

 elongates, and by the time of hatching two pairs of somites 

 have been constricted off (Fig. 9, A). The gastral mesoderm 

 is approximately limited to the region of these first two pairs 

 of somites, all of the posterior remainder forming from peri- 

 stomial mesoderm. 



As compared with the higher Chor dates the formation of 

 the. somites in Amphioxus begins very early, for in those forms 

 the mesoderm first separates from the endoderm as a pair of 

 longitudinal bands which later and only in part become divided 

 into segments or somites. The formation of the somites here, 

 as actual enterocoelic diverticula from the archenteron, has 

 been considered of great theoretic importance on account 

 of its supposed primitiveness, for in all of the Craniates the 

 mesoderm bands are at first solid and later develop a cavity 

 never directly continuous with the archenteron. It is quite 

 possible, however, that the formation of solid peristomial 

 mesocterm bands is the primary arrangement and that the 

 connection between the mesoderm cavity and the archenteron, 

 which we shall see is limited to the somites of gastral meso- 

 derm, is in reality a secondary condition. 



The first phase of embryonic development is terminated 

 about eight to fifteen hours after fertilization by the escape 

 of the embryo from the egg membranes, within which it has 

 been enclosed. During the remainder of the embryonic period 

 it swims freely near the surface by means of the covering of 

 long ectodermal cilia. We may conveniently summarize the 

 characters of the embryo at the time of hatching as follows 

 (Fig. 9, A). The embryo is in general cylindrical with a flat- 



