i6 THE PHYSIOLOGY OF TWINNING 



Neither one is fully normal, but both at least begin 

 gastrulation and some of them form fairly normal 

 gastrulae. Never, however, do these dwarfs attain a 

 condition that approximates that of a bipennaria larva. 

 They all stop at the gastrula stage. We may denominate 

 this mode of origin of twins physiological blastotomy; 

 for it is the result of the physiological isolation of the 

 first two blastomeres, so that they act as though they 

 were independent eggs. Such a condition is equivalent 

 to the occurrence of two separate blastoderms on a 

 fish or a chick egg. 



2. Twins with two or more archentera. — Much the 

 commonest mode of twinning in the starfish is one in 

 which two or more points of invagination occur, resulting 

 in two or more archentera. Many varieties of this 

 type of twin occur. A very common type is that in 

 which the original archenteron seems to persist and a 

 secondary one (often two or three) arise at the opposite 

 or apical end of the larva. Such a larva may sub- 

 sequently rid itself of these accessory archentera by 

 closing their blastopores and pinching off the small 

 archentera so as to leave one or more small internal 

 vesicles or cysts that remain attached to the body wall 

 for some time, only to be ultimately resorbed. Some 

 biaxiate larvae, however, such as that shown in Figures i 

 and 2, have the secondary archenteron nearly as large as 

 the primary and the animal exists as a biaxiate bipen- 

 naria for a long time. In swimming, such a larva moves 

 in a direction determined by the position of the primary 

 blastopore, i.e., with this structure posteriorly directed. 

 The anterior component of this twin seems to be under 

 the dominance of the posterior or primary component. 



