354 



THE CHORDATE BLASTULA AND ITS SIGNIFICANCE 



areas lie at the margins of the entodermal layer and surround it. As such, 

 some of the cells of these two, organ-forming areas may form part of the 

 floor of the blastula. The presumptive, notochordal and neural plate cells lie 

 at the future dorsal lip of the blastopore and form the dorsal crescent, while 

 the mesodermal area occupies the ventral-lip region as the ventral crescent 

 (fig. 167F). In Amphioxus, the mature blastula is pear shaped, with the body 



Fig. 168. Ultimate destiny within the developing body of presumptive organ-forming 

 areas of the late amphibian blastula, stained by means of vital dyes. (After Pasteels: J. 

 Exper. Zool., 89.) (A) Area of blastula, stained. (B) Destiny of cellular area, stained 

 in (A). (D, E) Ultimate destiny shown by broken lines of cellular areas, stained in 

 late blastula shown in (C). (E) Anterior trunk segment. (D) Posterior trunk segment. 



