GENERAL FEATURES OF CHORDATE DEVELOPMENT 



35 



its cells are laden with yolk. The cavity inclosed by the entoderm is the gastro- 

 coel. The opening of the archenteron to the exterior at the posterior end is 



blastopore 



arcbenteron 



archenteron 



ectoderm 



entoderm 



blastocoel 



imagination 



blastocoel 

 entoderm 



entoderm 



yolk 



FIG. 5. Gastrulation and formation of the entoderm in the three types of chordate eggs. All 

 shown in median sagittal section. A, Amphioxus; I, beginning of the in vagina tion; 2, invagination 

 completed; j, completed gastrula having a wall of two layers, ectoderm and entoderm, and an internal 

 cavity, the gastrocoel. B, amphibian; i, beginning of the invagination; 2, progress of the invagination 

 accompanied by downward growth of the ectoderm; j, completed gastrula, with very thick entoderm 

 ventrally. C, reptile or bird; i, posterior margin of the blastoderm beginning to turn under; 2, con- 

 tinuation of the invagination; a slight split between entoderm and yolk constitutes the gastrocoel. 

 In A 3, Bj, and 2 the anterior end of the embryo is to the left, posterior end to the right, dorsal surface 

 above, ventral below. (A from Parker and HaswelPs Textbook of Zoology, after Hatschek, courtesy of 

 the Macmillan Company; B from Kellicott's General Embryology, courtesy of Henry Holt and Com- 

 pany.) 



the blastopore. Ectoderm and entoderm are continuous at the rim of the blas- 

 topore. A portion of the entoderm, the yolk plug, protrudes through the 



