PORIFERA. 



and the cells of the greater part of the blastosphere become 

 columnar and ciliated, (fig. 64 en.} while the granular cells (ec.} 

 increase to about thirty-two in number and appear to be (parti- 

 ally at least) involuted into the segmentation cavity, reducing this 

 latter to a mere slit. This stage forms the last passed by the 

 embryo in the tissues of the parent. The general position of the 

 embryo while still in this situation may be gathered from fig. 64, 

 representing the embryo /// situ. The embryo is always placed 

 close to one of the radial canals. From this situation it makes 

 its way through the lining cells into a canal and is thence trans- 

 ported to the surrounding water. By the time the larva has 

 become free, the semi-invaginated granular cells have increased 

 in bulk and become everted so as to project very much more 

 prominently than in the encapsuled state. To the gastrula stage, 

 if it deserves the name, passed through by the embryo in the 

 tissues of the parent, no importance can be attached. 



The larva, after it has left the parental tissues, has an oval 

 form and is transversely divided into two areas (fig. 65 A). One 



cn\ 



cs 



FlG. 65. TWO FREE STAGES IN THE DEVELOPMENT OF SYCANDRA RAPHANUS. 

 (Copied from Schulze.) 



A. Amphiblastula stage. 



15. A later stage after the ciliated cells have commenced to become invaginated. 

 fs. segmentation cavity ; ec. granular cells which will form the epiblast ; en. 

 ciliated cells which become invaginated to form the hypoblast. 



of these areas is formed of the elongated, clear, ciliated cells, 

 with a small amount of pigment near their inner ends (en.}, and 



