THE DEVELOPMENT OF AMPHIOXUS 19 



They contribute also to the ectoderm, and thus cause a steady 

 extension of the dorsal lip posteriorly; that is, gastrulation is 

 accomplished in part by epiboly, or the growth and extension 

 backward of part of the blastoporal region. So that while at 

 first the invaginated blastula is only little more than hemi- 

 spherical in form, it soon begins to elongate, and this elonga- 

 tion extends chiefly posteriorly and is accomplished mainly 

 by the rapid posterior elongation of the dorsal margin of the 

 blastopore (Fig. 6, F-H). 



When the invaginating phase of gastrulation is completed 

 (Fig. 6, E) the archenteron has a nearly hemispherical form and 

 is widely open in the dorsal or postero-dorsal direction. In 

 Amphioxus the margin of the archenteric opening coincides with 

 the blastopore, though we have seen that in general the blasto- 

 pore should be regarded as that region where the ectoderm and 

 endoderm are continuous, whether this borders an opening or 

 not. As the dorsal margin of the blastopore extends backward 

 the diameter of its opening decreases, that is, the archenteron 

 becomes a more nearly enclosed cavity, considerably elongated 

 posteriorly. As the blastopore closes gradually the direction of 

 its opening becomes less dorsal and more posterior (Fig. 6, F, G). 

 Toward the end of gastrulation the sides as well as the dorsal 

 region of the blastoporal margin grow backward, and finally the 

 ventral region shares in the process so that the last stages in the 

 narrowing of the blastopore are accomplished by epiboly on 

 every side. The result of this is the formation in the endoderm, 

 just within the blastopore, of a band of cells, narrow below and 

 widening laterally, which have been formed differently from the 

 remainder of the ventral and lateral endoderm, and like the 

 endoderm forming dorsally. This has an important bearing 

 upon the development of later structures. 



At the close of gastrulation the embryo has the form shown in 

 Fig. 6, G. The gastrula is bilaterally symmetrical, quite elon- 

 gated antero- posteriorly, flattened dorsally, rounded ventrally as 

 well as anteriorly, while at the postero-dorsal aspect the archen- 

 teron opens directly to the outside by a narrow blastoporal 

 aperture. The ectoderm forms a fairly uniform layer of super- 



