THE BLASTULA IN RELATION TO TWINNING 



385 



area (fig. 187A), two embryos are produced as in figure 187 A'. However, if 

 the primitive node and primitive streak are split antero-posteriorly, as indi- 

 cated in figure 187B, two embryos, placed as in figure 187B', are produced. 



It is evident, therefore, that in the production of monozygotic twins, con- 

 dition (3) or the presence of the abihty to produce an organization center 

 is of greatest importance. In the case of the separation of the two blastomeres 

 of the two-cell stage in Amphioxus or of the division of the dorsal lip of the 

 early gastrula of the amphibian by a hair loop, as shown in figure 184, a 

 mechanical division and separation of the ability to produce an organization 

 center in each blastomere (Amphioxus) or of the separation into two centers 

 of the organization center already produced (Amphibia) is achieved. Once 

 these centers are isolated, they act independently, producing twin conditions, 

 providing the substrate is competent. Similar conditions evidently are pro- 

 duced in the duck-embryo experiments of Wolff and Lutz referred to above. 



In some teleost blastulae, e.g., Fundulus and Salmo, during the earlier period 

 of development, it has been found possible to separate the early biastoderm 

 into various groups of cells (Oppenheimer, '47) or into quadrants (Luther, 

 '36), and a condition of totipotency is established in each part. Totipotency 

 appears thus to be a generalized characteristic in certain teleost blastoderms 

 during the earlier phases of blastular development. Harmonious totipotency, 

 however, appears not to be achieved in any one part of the blastodisc of 

 these species during the early conditions of blastular formation. During the 



MODE OF 

 STREAK 



PRIMITIVE STREAK 

 AND NODE ARE CUT 

 INTO TWO HALVES 



Fig. 187. Isolation of the organization center in the early duck embryo. (From Dalcq, 

 '49, after Wolff and Lutz.) (A') Derived from blastoderm cut as in (A). (B') Derived 

 from blastoderm cut as in (B). 



