202 



Embryogenesis: Preparatory Phases 



die in an embryonic stage. Regulation to 

 diploidy is found to occur in a number of 

 ways depending on the species of animal, the 

 stage of maturation division of the egg at 

 the time of treatment, and the method of 

 treatment. Known methods of regulation in- 

 clude fusion of haploid nuclei at the first or 

 a later cleavage, fusion of egg nucleus with a 

 polar body nucleus, and utilization of a 

 polar spindle for cleavage. 



by different methods it is not knowoi to what 

 extent these represent differences in effective- 

 ness rather than failure to find the proper 

 exposure. Eggs of different groups of animals 

 frequently differ in respect to the kind of 

 agent to which they will best respond. As 

 yet no single generally effective method of 

 treatment has been described, nor have any 

 correlations been found between the effective 

 treatments for eggs of various species and 



Fig. 68. Diagram of the various types of behavior of the polar divisions and cleavage in Urechis eggs acti- 

 vated with anunoniacal sea water. Upper figure represents the unfertilized egg. A, B, CI, C2 and C3 represent 

 the types obtained with increasing time of treatment (e.g., with 0.01 N NH3 at 20° C, %-l min. -> type A; 

 1-10 min. -^ type B; 10-20 min. -^ types Ci, C2 and C3). Row / shows condition of egg at time when nor- 

 mally fertilized egg has extruded first polar body; row //, after time of second polar division; row ///, after 

 first cleavage time. The small broken circles represent nuclei within the egg; the other circles represent ex- 

 truded polar bodies. Eggs IllCi, IIIC2 and IIIC3 are figured in polar view, the others in side view. 



The type B egg, in which polar body formation is normal, fails to undergo cleavage. The other types con- 

 tinue to cleave and can develop into embryos of which many are normal. (After Tyler, '41.) 



Sex. In parthenogenetic frogs, silkworms 

 and rabbits the sex is found to accord, in 

 general, with the expectations on the basis 

 of whether the species is male or female 

 digametic and on the method of regulation 

 to diploidy. 



Methods of Activation. It is now well known 

 that a large variety of chemical and physical 

 agents may be used to activate eggs of vari- 

 ous animals. Even in a single species of 

 animals (e.g., the sea urchin Arbacia) the 

 eggs can respond to hypertonicity, hypoton- 

 icity, acids, bases, neutral salt solutions, 

 certain alkaloids, fat solvents, heat, cold, 

 puncture with a fine needle, ultraviolet radi- 

 ation and radium emanations. Although 

 there are differences in the results obtained 



properties or characteristics of the eggs. As 

 a result of the early discovery of the diversity 

 of methods that may be employed to activate 

 the egg the problem of activation has come 

 to be regarded as part of the general problem 

 of cell stimiilation. Theories of activation 

 have, then, centered about attempts to find 

 the common factor in the action of various 

 parthenogenetic agents. 



CURRENT THEORIES OF ACTIVATION 



The basis for Loeb's ('13) concept, that 

 activation resulted from the operation of a 

 cytolytic and a "corrective" factor, was 

 largely removed by Just's ('22, '30) demon- 

 stration that the "corrective" hypertonic 



