662 



MORPHOLOGY AND SYSTEMATIC 



granular cells takes place, reducing the segmentation cavity to a 

 mere slit; this invagination is, however, quite temporary and 

 unimportant, and on the embryo becoming free, which shortly 

 takes place, no trace of it is visible; but, on the contrary, the 

 segmentation cavity becomes larger, and the granular cells 

 project very much more prominently than in the encapsuled 

 state. 



FIG. i. 



en 



en\ 



e.g. 



Two free stages in the development of Sycandra raphanus (copied from Schulze). 



A. Amphiblastula stage; B, a later stage after the ciliated cells have commenced to 

 become invaginated ; cs. segmentation cavity ; ec. granular cells which will form 

 the ectoderm; en. ciliated cells which become invaginated to form the entoderm. 



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

 form and is transversely divided into two areas (fig. I, A). One of 

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

 a small amount of pigment near the inner ends (en}, and the 

 other and larger area of the thirty-two granular cells already 

 mentioned (ec). Fifteen or sixteen of these are arranged as a 

 special ring on the border of the clear cells. In the centre of 

 the embryo is a segmentation cavity (cs) which lies between the 

 granular and the clear cells, but is mainly bounded by the vaulted 

 inner surface of the latter. This stage is known as the amphi- 

 blastula stage. After the larva has for some time enjoyed a 

 free existence, a remarkable series of changes takes place, which 

 result in the invagination of the half of it formed of the clear 



