THE DEVELOPMENT OF VERTEBRATA. 233 



It was formerly thought that an actual invagination took 

 place at the blastopore, to form the walls of the mesenteron, 

 This is a mistake, and would be strange if it were true, 

 since we are by this time obviously past the stage at which 

 invagination occurred in Amphioxus. The cells immediately 

 around the blastopore do, however, show characters inter- 

 mediate between the epiblast and yolk- cells, and it seems 

 that during the next stages of growth they give rise to cells 

 of all three layers (epi-, meso-, and hypo-blast) : the same is 

 probably true of the cells around the blastopore of 

 Amphioxus. 



Why have we called the cavity formed in the mass of 

 yolk-cells mesenteron and not archenteron ? In Amphioxus 

 we had an archenteron, whose walls were primitive 

 hypoblast : the cavity became divided into mesenteron and 

 myoctelomic cavities, while the primitive hypoblast differ- 

 entiated into hypoblast, notochord and mesoblast. In the 

 frog, the yolk-cells, after the supplementary epiblast has 

 been separated off from them, correspond precisely with the 

 primitive hypoblast, and, like that, differentiate into 

 hypoblast, notochord and mesoblast. But the mesenteron 

 and myoccelomic cavities both appear by splitting in a solid 

 mass of cells, and are never connected with one another, so 

 that there is never any actual archenteron-cavity in the 

 frog. 



7. The Origin of the Three Germinal Layers in the 

 frog may now be stated. 



The epiblast is formed partly from the primitive epiblast 

 cells of the animal hemisphere, and partly from supple- 

 mentary epiblast-cells divided off the yolk- cells. 



The hypoblast is formed of the remainder of the yolk-cells 

 after (1) the supplementary epiblast and (2) the mesoblast, 

 have been differentiated from them. 



The mesoblast is formed from yolk- cells. Its differentia- 

 tion begins near the dorsal surface, and gradually extends 

 to the ventral side. 



8. Neural Folds and Notochord. In Amphioxus the 

 gastrula stage was followed by one in which the neural plate 



