TWINNING IN BIRDS 79 



approach each other toward the center of the blasto- 

 derm. Sometimes the two embryos remain entirely 

 separate, at least for a time. Kaestner (1901) figures 

 a twin chick in the primitive streak stage in which 

 the two axes were in the same line, much like the twin 

 starfish shown in Figure 6, but the anterior ends are still 

 some distance apart. The same author presents a very 

 clear photograph (Fig. 36) of a similar pair of twin 

 embryos in a thirteen- or fourteen-somite stage, with 



Fig. 36. — A rare type of chick twins that are the product of double, 

 symmetrical gastrulation. The two embryonic axes have grown inward 

 toward each other and have barely avoided a collision such as has taken 

 place in the twins shown in Fig. 39. (After Kaestner.) 



axes almost in the same line. Their heads have, how- 

 ever, shoved past each other as far as the hind-brains. 

 Though there is some contact and confusion in the 

 membranes, especially the amnia, of these twins, they 

 are entirely separate and have, in my opinion, arisen as 

 two quite distinct twin areas of gastrulation on opposite 

 sides of the blastoderm. This opinion is strengthened 

 by a study of a very interesting double monster in my 

 possession (Fig. 37). In this case it is obvious, I beheve, 



