20 



THE PHYSIOLOGY OF TWINNING 



discussed in a subsequent section under the caption An 

 adidymi among starfishes (pp. 23, 24). 



The mode of origin of these twins resulting from 

 plural gastrulation processes is of considerable interest. 

 One can easily trace the twinning back to the blastula 



// 



^J 



Figs. 9-14. — ^Types of blastulae with appropriate gastrulae of Patiria. 

 Fig. 9, a typical blastula, with the typical gastrula (Fig. 10) that arises 

 from it. Fig. 11, a blastula that has entirely lost its polarity, with the 

 very abnormal multiple gastrula (Fig. 12) that arises from it. Fig. 13^ 

 a bipolar blastula, the original polarity of which has been only partially 

 lost, with the bipolar gastrula (Fig. 14) that arises from it. (From 

 Newman.) 



stage. A normal starfish blastula has a very definite 

 primary polarity. As shown in Figure 9, the blastula 

 wall is much thicker on one side than the other. The 

 cells of the thicker region are loaded with yolk granules 



