THE DEVELOPMENT OF AMPHIOXUS 15 



3. Gastrulation 



By the time about two hundred and fifty-six cells have been 

 formed in the blastula, or about four hours after fertilization, 

 the- process of gastrulation is commenced, by which the single 

 walled blastula is to be converted into a double walled gastrula. 

 We should notice in advance that the gastrula of Amphioxus 

 is formed by the three processes of invagination, involution, and 

 epiboly, and so is not typical of Chordates generally, in which 

 the double walled or two layered condition results more 

 extensively from delamination, that is, by tangential divisions 

 in the wall of the blastula, together with some involution and 

 epiboly. 



The first indication of gastrulation is the flattening of the 

 vegetal pole of the blastula (Fig. 6, A), which is soon followed 

 by the appearance of a slight infold on the antero-dorsal 

 aspect, at about the level of the equator (Fig. 6, B). This- 

 infolding or invagination soon extends around the sides of the 

 blastula, and finally the whole flattened vegetal or postero- 

 dorsal region becomes folded down into the segmentation 

 cavity (Fig. 6, C). The process of infolding continues in a 

 more advanced stage in the antero-dorsal region where it com- 

 menced, and here the folded layer first comes into contact with 

 the inner surface of the cells of the animal pole so as to obliterate 

 the blastoccel there, at a time when this remains quite widely 

 open elsewhere. We may now speak of the invaginating and 

 non-invaginating layers as endoderm (hypoblast) and ectoderm 

 (epiblast) respectively, and of the whole structure as the 

 gastrula (Fig. 6, E). While the infolding of the endoderm 

 leads to the obliteration of the blastoccel, it leads to the forma- 

 tion of another cavity which is lined completely with cells of 

 one kind endoderm. This cavity, which can be recognized 

 as beginning with the earliest infolding of the blastula wall, is 

 the archenteron or primitive gut cavity, which is to give rise to 

 the chief cavities of the later embryo. In many Chordates 

 this cavity is virtual rather than real on account of being 

 filled with yolk-cells. The gastrula of Amphioxus is often 



