PORIFERA. 



115 



This stage nearly completes the segmentation: in the next one, the 

 cells of the poles of the blastosphere increase in number, and the cells 

 of the greater part of the blastosphere become columnar and ciliated, 



FIG. 64. LABVA OF SYCANDRA EAPHANUS AT PSEUDOGASTRULA STAGE, IN SITU IN THE 

 MATERNAL TISSUES. (Copied from F. E. Schulze.) 



me. mesoblast of adult; hy. collared cells forming hypoblast of the adult; en. clear 

 cells of larva which eventually become involuted to form the hypoblast; ec, granular 

 cells of larva which give rise to the epiblast, which at this stage are partially involuted. 



(fig. 64 en.) while the granular cells (ec.) increase to about thirty- 

 two in number and appear to be (partially 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 in 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 transported 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 of these 

 areas is formed of the elongated, clear, ciliated cells, with a small 



82 



