382 



THE BLASTULA IN RELATION TO INNATE CONDITIONS 



CENTER OF 

 (EMBRYO N I 



R G A N IZ AT 

 SHIELD) 



Fig. 185. Twinning in teleost fishes. (After Morgan, '34; Embryology and Genetics, 

 Columbia University Press, pp. 102-104. A, B, C from Rauber; D from Stockard.) In 

 certain teleost fishes, especially in the trout, under certain environmental conditions, 

 two or more organization centers arise in the early gastrula. (A-C) These represent 

 such conditions. If they lie opposite each other as in (A), the resulting embryos often 

 appear as in (D). If they lie nearer each other as in (B) or (C), a two-headed monster 

 may be produced. 



has been described in Chapter 6. In these cases all the conditions mentioned 

 above are fulfilled. However, in the case of the isolation of the first two blas- 

 tomeres in Styela described in Chapter 6, evidently conditions (1), (2), and 

 (3) are present in each blastomere when the two blastomeres are separated, 

 but (4) is absent and only half embryos result. That is, each blastomere has 

 been determined as either a right or left blastomere; with this determination 

 of potency, the power for regulation is lost. In the frog, if the first two blasto- 

 meres are separated when the first cleavage plane bisects the gray crescent, 

 all four conditions are present and two tadpoles result. If, however, the first 

 cleavage plane separates the gray-crescent material mainly into one blastomere 

 while the other gets little or none, the blastomere containing the gray-crescent 

 material will be able to satisfy all the requirements above, and it, consequently, 

 develops a normal embryo. However, the other blastomere lacks (2), (3), 

 and (4) and, as a result, forms a mere mass of cells. Again, animal pole 

 blastomeres, even when they contain the gray-crescent material, when sepa- 

 rated entirely from the yolk blastomeres, fail to go beyond the late blastular 

 or beginning gastrular state (Vintemberger, '36). Such animal pole blasto- 

 meres appear to lack requirements (1), (2), and possibly (3) above. Many 

 other illustrations of embryological experiments could be given, establishing 



