TWINNING IN BIRDS 



77 



many kinds have been published. A survey of a large 

 niunber of these figures and of sections through crucial 

 parts of double embryos, together with a study of a 

 collection of several pairs of chick twins in my own 

 possession, has convinced me that the situation is 

 essentially the same as that described for the star- 

 fish; that the modes of twinning and the different 

 categories of twins are essentially the same in two 

 cases. I find in the birds the following kinds of twins: 

 (i) those derived from two blastoderms on a single 

 yolk; (2) those derived from two embryonic axes on 

 one blastoderm; (3) those derived by the isolation, 

 more or less complete, of the bilateral halves of a single 

 embryonic axis; (4) those derived by the growing 

 together of two separate embryonic axes; and (5) those 

 in which one compo- 

 nent of a double mon- 

 ster is more or less 

 completely absorbed 

 by the other. These 

 five categories of 

 twins will be discussed 

 seriatim. 



I. That separate 

 blastoderms on a 

 single yolk actually 

 do occur seems cer- 

 tain in view of the 

 clear figures of Da- 

 reste and of Kaestner. 

 That of Dareste 

 (Fig. 34) represents a 



Fig. 34. — A case of triplets in which 

 the upper individual probably arose from 

 an early fission of the blastoderm into two 

 unequal masses. The lower pair of twins 

 is obviously the result of double gastru- 

 lation of the larger moiety of the divided 

 blastoderm. (After Dareste.) 



