XXV 



FORMATION OF TROCHOSPHERE 



295 



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

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

 stage of development is called the blastula. 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 india-rubber ball. The resulting embryonic stage 



FIG. 72. Diagram illustrating the origin of the trochosphere from 

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



A, gastrula, with enteron (Ent) and gastrula-mouth (Cast. Mth\ and 

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

 blastocoele (Bl. cccl}. 



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

 sac. 



C, two ectodermal pouches, the stomodseum (St. dm] and proctodseum 

 (Prc. dm) have appeared. 



D, the stomodaeum (St. dm) and proctodseum (Prc. dm) have opened 

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

 (Mth) and anus (An). 



is known as the gastmla : 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. Mtti). Between the ectoderm 

 and endoderm is a space, the greatly diminished blastoccele. 

 The resemblance of the gastrula to a simplified Hydra, 

 devoid of tentacles, will be at once apparent 



