396 



HEMICHORDA OR ENTEROPNEUSTA. 



Development. 



The eggs must be fertilised outside of the body. Seg- 

 mentation is complete and approximately equal ; a blasto- 

 sphere or blastula results ; this is invaginated in the normal 

 fashion, and becomes a two-layered gastrula. 



The American species (B. kowalevskii) has a simpler 

 development than the others, for it is without a remarkable 

 larval form (Tornaria) which occurs in them. We shall take 

 the simpler case first, though it is probably less primitive. 



The blastopore or mouth of the gastrula narrows and 

 closes; the external surface of the gastrula becomes ciliated; 



FIG. 126. Development of Balanoglossus. (After BATESON. ) 



The mesoderm is represented by the broken dark line. 

 In the upper row, from the left 



Section of blastula ; beginning of gastrulation, End, endoderm ; 

 section of gastrula, 3/, blastopore; Ac, Archenteron ; Sc, 

 segmentation cavity ; closure of blastopore, outgrowth of five 

 coelome pouches (M). 

 In the lower row, from the left 



Longitudinal section, showing the five parts of the body cavity 



(&ri, &T2, ^3,) or coelome. 

 Cross section, CNS, central nervous system ; Nch, notochord ; 



bcz, body cavity in collar region. 

 Section at a later stage, D.b.v., dorsal blood vessel. 



the endoderm lies as an independent closed sac within the 

 ectoderm. Meanwhile the embryo has become or is becom- 

 ing free from the thin egg envelope, and begins to move 

 about at the bottom in shallow water. It elongates and 

 becomes more worm-like; there is an anterior tuft and a 



