3i6 



THE STARFISH 



gonads are all connected by cords of tissue with an organ 

 called the axial organ, which lies alongside the stone-canal 

 and is surrounded by a blood-sinus. Its function is not 

 known with certainty. 

 The ova and sperms are shed into the water, where im- 



FlG. 79,— Early stages in the development of a Starfish. 



A. The polyplast, surrounded by the vitelline membrane. 



B. The blastula, in section. 



C The gastrula, external view, showing the blastopore [bl. p). 



D. The gastrula, in vertical section : arch, enteron. 



E. More advanced gastrula, with ciliated ectoderm. 



Arch, enteron ; blastoc, blastocoele ; bl.p. blastopore ; cct, ectoderm ; 

 end, endoderm. 



(From Parker and Haswell's Zoology.) 



pregnation takes place. The oosperm undergoes the usual 

 process of segmentation, forming a polyplast (Fig. 79, a), 

 which is soon converted into a blastula (b) by the cells arrang- 

 ing themselves round a central cavity. One side of the 

 blastula becomes invaginated or tucked in, and a gastrula 



