80 



ORGANISMS OF TISSUES 



formed directly from this enteric cavity. Because of this ulti- 

 mate fate the cavity is called the archenteron or primitive gut, 

 while the opening to the outside is termed the llastopore or larval 

 mouth. At this stage in development the water dwelling types 

 usually leave the egg membrane and swim about in the water, 

 taking in food through the blastopore and digesting it in the 

 archenteric cavity. We see, therefore, that the larger cells of 



EARLY CLemacE STAGES 



FIG. 33. Holoblastic cleavage and gastrulation of Amphioxus. P.B., polar body 

 (see p. 213). (From Ziegler models.) 



the vegetative pole of the blastula become the functioning cells 

 of the digestive tract of the adult and are internal in position, 

 forming a lining of cells called the hypoblast or endoderm. The 

 cells of the animal pole remain outside, covering the endoderm 

 cells and forming a continuous sheet of cells termed the epiblast 

 or ectoderm. The two layers together constitute the primary 

 germ layers of the organism. At first each sheet consists of 



