384 THE BLASTULA IN RELATION TO INNATE CONDITIONS 



the necessity for the presence of all the above conditions. Successful whole 

 embryos have resulted in the amphibia when the two-cell stage and beginning 

 gastrula is bisected in such a manner that each half contains half of the chorda- 

 mesodermal field and yolk substance; that is, each will contain half of the 

 organization center (fig. 184). 



Monozygotic twinning occurs occasionally under normal conditions in the 

 teleost fishes. In these cases, separate centers of organization arise in the 

 blastoderm, as shown in figure 185. When they arise on opposite sides of 

 the blastoderm, as shown in figure 185A, twins arise which may later become 

 fused ventrally (fig. 185D). When the centers of organization arise as shown 

 in figure 185B, C, the embryos become fused laterally. Stockard ('21) found 

 that by arresting development in the trout or in the blastoderm of Fundulus 

 for a period of time during the late blastula, either by exposure to low tem- 

 peratures or a lack of oxygen, twinning conditions were produced. The arrest 

 of development probably allows separate centers of organization to arise. 

 Normally, one center of organization makes its appearance in the late blastula 

 of these fishes, becomes dominant, and thus suppresses the tendency toward 

 totipotency in other parts of the blastoderm. However, in the cases of arrested 

 development, a physiological isolation of different areas of the blastoderm 

 evidently occurs, and two organization centers arise which forthwith proceed 

 to organize separate embryos in the single blastoderm. Conditions appear more 

 favorable for twinning in the trout blastoderm than in Fundulus. After the 

 late blastular period is past and gastrulation begins, i.e., after one organization 

 center definitely has been established, Stockard found that twinning could not 

 be produced. 



In the Texas armadillo, Tatusia novemcincta, Patterson ('13) found that, in 

 the relatively late blastocyst (blastula), two centers of organization arise, and 

 that, a little later, each of these buds into two separate organization centers, 

 producing four organization centers in the blastula (fig. 186A-C). Each of 

 these centers organizes a separate embryo; hence, under normal conditions, four 

 embryos (polyembryony) are developed from each fertilized egg (fig. 186D). 



It is interesting in connection with the experiments mentioned by Stockard 

 above, that the blastocyst (blastula) in Tatusia normally lies free in the uterus 

 for about three weeks before becoming implanted upon the uterus. It may 

 be that this free period of blastocystic existence results in a slowing down of 

 development, permitting the origin of separate organization centers. In har- 

 mony with this concept, Patterson ('13) failed to find mitotic conditions in 

 the blastoderms of the blastocysts during this period. 



In the chick it is possible to produce twinning conditions by separating 

 the anterior end (Hensen's node) of the early primitive streak into two parts 

 along the median axis of the developing embryo. Twins fused at the caudal 

 end may be produced under these conditions. In the duck egg, Wolff and Lutz 

 ('47) found that if the early blastoderm is cut through the primitive node 



