FORMATION OF TROCHOSPHERE 



295 



in its interior, into a hollow sphere, bounded by a single 

 layer of cells and containing a cavity, the biastocoek: this 

 stage of development is called the hlastula. Next, one side 

 of the blastula becomes tucked in or invaginated so as to 

 convert the embryo from a single-layered sphere into a 

 double-layered cup (Fig. 72, a). This process can be 

 sufficiently well imitated by pushing in one side of 

 a hollow mdia-rubber ball. The resulting embryonic stage 



/y static _ ^^ ,, V X  , , , , 



'Pre. dm Xtjljy 

 GastMik \l» 



Fig. 72. — Diagram illustrating the origin of the trochosphero from 

 the gastrula. The ectoderm is dotted, the endoderm striated. 



A, gastrula, with enteron {Eiit) and gastrula-mouth {Cast. Mlh), and 

 with the ectoderm and endoderm separated by the larval body-cavity or 

 blastocoele {Bl. ca-l). 



B, the gastrula-mouth has closed, the enteron [Ent) becoming a shut 

 sac. 



c, two ectodermal pouches, the stomoda;um {St. din) and proctodseum 

 (Pre. dm) have appeared. 



D, the stomodieum {St. dm) and proctodoeum {Pre. dm) have opened 

 into the enteron {Ent), forming a complete enteric canal with mouth 

 {Mth) and anus {An). 



is known as the gastrula : its cavity is the enteron {Eni) and 

 is bounded by the invaginated cells which now con- 

 stitute the endoderm, the remaining cells, forming the outer 

 wall of the gastrula, being the ectoderm. The two layers 

 are continuous at the aperture of the cup, the gastrula- 

 mouth or blastopore {Gast. Mth). Between the ectoderm 

 and endoderm is a space, the greatly diminished blastocoele. 

 The resemblance of the gastrula to a simplified Hydra, 

 devoid of tentacles, will be at once apparent 



