PRODUCTION OF TWINS AND DOUBLE-MONSTERS 



331 



and one or more secondary archentera. If we use figure 19 as 

 a norm representing a typical gastrula, we may note the various 

 types of larvae with plural archentera. Figure 23 shows an almost 

 normal larva, atypical only in the presence of a second thickened 

 region at the apical end of the larva. Figure 24 shows a common 



14 V— ^^ ^^-^ 16 



Fig. 13 Outline drawing of a blastula without axiate organization. 



Fig. 14 A gastrula with multiple archentera derived from a blastula like that 

 in figure 13. 



Fig. 15 Blastula with two basal regions, a primary and a secondary. 



Fig. 16 A gastrula with two archentera, the secondary forming at the apical 

 end. 



Fig. 17 



Fig. 18 



A blastula with three basal areas. 



A gastrula with a primary and two secondary archentera resulting 



from a larva like that in figure 17. 



condition in which this thickened apical plate has undergone a 

 slight, but definite invagination. Figure 21 shows a larva with 

 three small, but very distinct, secondary archentera at the apical 

 end of the larva. Figure 22 represents a much more distorted 

 type of twin larva in which the secondary archentera, one on the 

 side and one at the apical end, are large, though considerably 

 smaller than the primary archenteron. Figure 20 is a rare, type 



