107 



ghardaquana (Mortensen, 1938). In this sea star cytotomy (cell division) is 

 preceded by repeated division of the nuclei; the latter then migrate to the 

 periphery of the egg and initially form a single layer that becomes multilay- 

 ered. Separation of the cellular boundaries in the embryo of F. ghardaquana 

 continues for a long time. Delayed cytotomy, compared to nuclear division, 

 characterizes the large eggs of Henricia saguinolenta. In the deepwater 

 Aspidodiadema jacobyi, at the blastula stage the egg yolk lies under the 

 blastoderm (Young and Cameron, 1987). In many sea stars (especially in 

 astropectinids and many asterinids), there are surface folds on the morula or 

 blastula which are smoothed with the emergence of the blastula from the 

 vitelline membrane. Shedding of the membrane, i.e., changing over to a free 

 larval life, occurs very early in echinoderms and especially in sea stars. The 

 first larval stage is the late blastula in some species of sea stars or the early 

 gastrula in others. 



Gastrulation : Gastrulation begins soon after hatching or not long before 

 it. It is preceded by some flattening of the blastular walls at the vegetal pole 

 (Figure 66). Invagination of the cell layer begins at the site of flattening 

 (Figure 67). A long narrow archenteron forms and from its base expulsion 

 of the mesenchymal cells begins, which have long filopodia touching the cells 

 in the wall of the blastocoel (Figure 68). Later, the archenteron becomes 

 mushroom-shaped. The cavity of the "stalk" is lined with endodermal cells 

 and the cavity of the "cap" (the future coelomic cavity) is bordered by 

 mesodermal cells. The primordium of the mesoderm divides into two closed 



Figure 67: Gastrula of Patiria pectinifera. 



Figure 68: Archenteron of the gastrula of 

 Pisaster ochraceus (from Chia, 1977). 



