380 THE BLASTULA IN RELATION TO INNATE CONDITIONS 



differentiation (see p. 375). On the other hand, the notochord, mesodermal 

 area possesses both competence and the ability for self-differentiation. 



Competence appears to be a function of a developmental time sequence. 

 That is, the time or period of development is all important, for a particular 

 area may possess competence only at a single, optimum period of develop- 

 ment. The word competence is sometimes used to supersede the other terms 

 of potency or potentiality (Needham, '42, p. 112). 



D. The Blastula in Relation to Twinning 



1. Some Definitions 

 a. Dizygotic or Fraternal Twins 



Fraternal twins arise from the fertilization of two separate eggs in a species 

 which normally produces one egg in the reproductive cycle, as, for example, 

 in the human species. Essentially, fraternal twins are much the same as the 

 "sibhngs" of a human family (i.e., the members born as a result of separate 

 pregnancies) or the members of a litter of several young produced during 

 a single pregnancy in animals, such as cats, dogs, pigs, etc. Fraternal twins 

 are often called "false twins." 



b. Monozygotic or Identical Twins 



This condition is known as "true twinning," and it results from the devel- 

 opment of two embryos from a single egg. Such twins presumably have an 

 identical genetic composition. 



c. Polyembryony 



Polyembrony is a condition in which several embryos normally arise from 

 one egg. It occurs regularly in armadillos (Dasypopidae) where one ovum 

 gives origin normally to four identical embryos (fig. 186). 



2. Basis of True or Identical Twinning 



The work of Driesch (1891) on the cleaving, sea-urchin egg and that of 

 Wilson (1893) on the isolated blastomeres of Amphioxus mentioned above 

 initiated the approach to a scientific understanding of monozygotic or identical 

 twinning. Numerous studies have been made in the intervening years on the 

 developing eggs of various animal species, vertebrate and invertebrate, and 

 from these studies has emerged the present concept concerning the matter of 

 twinning. True twinning appears to arise from four, requisite, fundamental, 

 morphological and physiological conditions. These conditions are as follows: 



(1) there must be a sufficient protoplasmic substrate; 



(2) the substrate must contain all the organ-forming stuffs necessary to 

 assure totipotency, that is, to produce all the necessary organs; 



