1907.] 



Development of Ophiothrix fragilis. 



441 



on that occasion, but on other occasions an abundant supply of the early 

 stages of development was obtained, so that the gap in the series of larvae 

 captured in the Plankton was completely filled. 



If artificial fertilisation is carried out in the usual way, viz., by dissecting 

 oat the ovaries and shaking them over a beaker of clean sea-water in order to 

 let the -eggs fall out, and then shaking in the water a piece of testis extracted 

 from a ripe male, the development takes place in the manner which I have 

 already described to the American Society of Zoologists (1), and which is 

 illustrated in figs, la, lb, and lc. The egg segments so as to give rise to 

 a solid mass of cells or morula. 



Fig. 1. 



'-■Mes. 



Ic. 



Id. Ie. If. 



Figs, la, b, c. Three stages in the abnormal development of Ophiothrix fragilis. 



Mes., mesenchyme. Cm., coelomic rudiment. 

 Figs. Id, e,f. Three stages in the normal development of Ophiothrix fragilis. 



When the embryo escapes from the egg-membranes and commences to lead 

 a free-swimming existence it has an ellipsoidal form, and consists of an outer 

 mantle of columnar ciliated cells and of an inner mass of rounded cells. It 

 very much resembles the "planula" larva of a Coelenterate. Subsequently an 

 invagination takes place at one pole of the egg, and the archenteron is formed. 

 But the invaginated cells do not form a single layer. A multiplication of 

 cells takes place at the vegetable pole before the invagination takes place, 

 and when this occurs it takes place in such a manner as to give rise to a sac 

 with a tongue of cells projecting into the lumen. A similar development in 

 the case of Ophiura brevis has been described by Grave (2). 



The larva soon assumes a triangular form (fig. lc), one angle being the 



