104 



F. M. BALrOUR. 



the form which was studied by both Metschnikoff and Schulze. 

 The segmentation of the ovum, though in many ways remark- 

 able, is of no importance for my present purpose, and I take up 

 the development at the close of the segmentation, while the 

 embryo is still encapsuled in the parental tissues. It is at this 

 stage lens-shaped, with a central segmentation cavity. An 

 equatorial plane divides it into two parts, which have equal shares 

 in bounding the segmentation cavity. One of these halves is 

 formed of about thirty-two large, round, granular cells, the other 

 of a larger number of ciliated clear columnar cells. While the 

 embryo is still encapsuled a partial invagination of the 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. 



Tig. 1. 



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



Schulze). 

 A. Amphiblastula stage ; B, a later stage after the ciliated cells have com- 

 menced to become invaginated ; cs. segmentation cavity ; ec. granular 

 cells which will form the ectoderm ; en. ciliated cells which become in- 

 vaginajted 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. 1, 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 mentione d 

 {ec) . Fifteen or sixteen of these are arranged as a special ring o n 



