Hyhridization of Echinoids. 17 



and we might distinguish between early developmental block and 

 late developmental block. In the material described in this paper, 

 and in much material of a similar nature, normal development stops 

 at the beginning of gastrulation, even though there has been no 

 evident failure in any of the processes of development up to that time. 



It has been found possible to overcome cortical block by various 

 preparatory treatments of the egg, and some success has been won in 

 the search for methods of eliminating internal block, but little has 

 been accomplished in the way of devising corrective methods for 

 block of the later period. 



Godlewski's (1911) method of treatment of Sphcerechinus eggs 

 after insemination with Chcetopterus sperm is an excellent example 

 of success in the elimination of internal block. Godlewski found it 

 possible to activate the eggs with the foreign sperm. The fertilization 

 membrane was formed, but no further development followed. After 

 determining that development did not follow when unfertilized eggs 

 were subjected to a short treatment with hypertonic sea-water, he 

 found that if the eggs were first inseminated with the foreign sperm 

 and then subjected to a treatment of 22 to 25 minutes with hyper- 

 tonic sea-water, the sperm and egg nuclei united and the eggs con- 

 tinued to the pluteus stage. Following this union of the nuclei, the 

 paternal chromatin was eliminated and the later development was in 

 effect parthenogenetic. A similar method of treatment was not 

 successful with Sphcerechinus eggs inseminated with Dentalium 

 sperm. 



Cross- Activation versus Cross-Fertilization. 



Among investigators in the field of experimental hybridization, 

 there has long been dissatisfaction with the term "cross-fertilization," 

 for the reason that it does not always describe the nature of the 

 result obtained. The term may or may not be correctly applied. 

 The phrase "attempt at cross-fertilization" does not remove the 

 difficulty. If one speaks of successful cross-fertilization, the difficulty 

 is even greater. In one paper (1911) I arbitrarily designated as 

 "successful" those crosses that gave swimming gastrulae. That was 

 for the purpose of ruling out all cases which resulted in the develop- 

 ment of irregular, formless masses of cells. Yet "successful fertiliza- 

 tion" can not be a relative term. The only test of success in fertili- 

 zation is in the production of fertile offspring. This might be called 

 the eugenic test. 



If we regard fertilization as the cortical reaction of the egg to 

 contact with the spermatozoon, cross-fertilization would follow 

 many inseminations, and the term "cross-fertilization," as used com- 

 monly in the biological literature of the present time, would not be 

 incorrect. 



