14 DR. J. F. GEMMILL ON THE DEVELOPMENT OF 



13 hours after shedding . . . The first cleavage completed. 



15 „ ,, ... The second cleavages completed. 



18 ,, ,, ... The third cleavages completed. 



20 „ „ ... Egg divided into about 16 cells. 



38 „ „ ... „ „ „ 64 „ 



2^ days after shedding... Segmentation completed; a solid "morula" stage made up of 

 quite small cells. 



4 „ „ ... Blastula shoTring numerous furrows externally. 



5 „ „ ... Only a few furrows now visible ; gastrulation beginning. 



6 „ „ ... Gastrulation complete. 



(For later chronology, see pp. 17, 25.) 



IV. External Charactees, Movements, &c. during the Free-swimming Period. 

 (PL I. figs. 12-14 ; PL II. figs. 15-20.) 



In describing the succeeding stages in the development ot Solaster I think it right 

 to make use of the terms larva and larval (as Masterman does for Cribrella), although 

 a larval mouth is never formed and the store of food contained in the egg has to last 

 until the end of metamorphosis. Comparison vpith other forms, particularly with 

 Asterina and Cribrella, enables one already to fix the anterior and posterior ends of 

 the larva. The former is indicated by the summit of the gastrula-dome and the latter 

 by the blastoporic opening. 



The dome-shaped gastrula gradually elongates to form a cone, the base of v^hich is 

 perforated by the blastopore (PL II. fig. 15j. The apex of the cone (anterior end of 

 the larva) next makes a slight bend towards what is to be the ventral aspect (fig. 16). 

 A ventral concavity or dimple is thus formed near the junction of the anterior 

 and middle thirds of the cone. All the larval planes and surfaces can now be readily 

 identified (see p. 37). The three larval arms next begin to grow out around the 

 dimple above named. All three are directed ventrally. There is an anterior 

 median unpaired one in front. The other two form a pair, one on each side a short 

 distance behind the dimple. A little later the cells forming the floor of the 

 dimple become elongated to form a disc-shaped sucker. The part of the cone which 

 carries the arms and the sucker is recognizable afterwards as the preoral lobe of 

 the larva. 



Meantime certain changes have become manifest in the posterior part of the cone, 

 which may now be called the body of the larva. This part becomes flattened from 

 side to side and correspondingly extended in the sagittal plane (figs. 16-18). The 

 blastopore lessens in size and the base of the cone becomes slightly oblique (p. 37). 

 Dorsally on the right side, a little behind the junction of the preoral lobe and the 

 body of the larva, a pit makes its appearance, at the bottom of which the hydroporic 

 opening is formed. As will be seen later (p. 26), this takes place through the sinking 

 in of a short ectodermal funnel to meet a hollow outgrowth from the anterior coelom. 



