TWINNING IN BIRDS 85 



latter is believed to be the equivalent of the closed blasto- 

 pore and its anterior end is the dorsal lip region, equiv- 

 alent to that described as a secondary point of high 

 susceptibility in the frog. It appears that either one of 

 these regions may undergo fission without the other or 

 that both may undergo fission more or less completely. 

 Kaestner (1898) describes and figures an interesting 

 case of a double chick embryo in approximately the same 

 stage as that of Tannreuther shown in Figure 41, but 

 the head, instead of being a normal single region, is 

 much broader than usual and is partially double, the 

 forebrain being single and the rest of the central nervous 

 system being double. Such a condition is interpreted 

 as a case of partial fission of the head-process region of 

 the blastoderm. Whether it is possible for two entirely 

 separate chick embryos to arise through the fission of 

 the bilateral primordia of a single embryonic axis is a 

 question at present unsettled. Tannreuther shows an 

 example of twin chicks that I am compelled to interpret as 

 products of a nearly complete fission. As may readily be 

 seen (Fig. 42, p. 86) the two embryos are entirely separate 

 except at the very posterior end where, back of the tail- 

 bud region, the two primitive streaks unite for a short 

 distance. Unless some sort of gross concrescence of the 

 posterior margin of the blastoderm has occurred, this 

 single region of the primitive streak can be explained 

 only as the residue of an originally single embryonic axis 

 that has undergone almost complete bilateral fission. In 

 this blastoderm the two embryos are markedly different 

 in size, the right-hand one being much larger and more 

 advanced (fourteen somites), while the smaller one is in 

 a ten-somite stage. Tannreuther is of the opinion, and 



