Echinoderm Hybridization. 123 



In another paper (Tennent 1910a) I have described in detail the 

 methods that were employed in my work at Beaufort. Similar care 

 was exercised in the work at Tortugas in order to avoid results which 

 might be due to careless laboratory manipulation. All dishes and 

 instruments used were sterilized and especial care to avoid chance 

 fertilization was taken. 



AIM OF THE INVESTIGATION. 



The aim of the research is the formulation of a statement of the 

 conditions governing the resemblance of the offspring to the parent. 



Herbst (1906 a, p. 173) has stated the purpose of his investigations 

 simply when he asks the question, "Why do the offspring sometimes 

 stand as a mean between the two parents; why do they sometimes 

 incline more to one or more to the other; or again, why do they resemble 

 one parent completely or almost completely while the characters of 

 the other are repressed?" 



We are acquainted to some extent with three types of heredity: 

 (i) Blended heredity. (2) Particulate or Mosaic heredity. (3) Alter- 

 native heredity. In the vast amount of research that is being done 

 to-day we lose sight of the fact that this is simply an artificial distinction 

 and that the actual aim of studies of this kind is not the elucidation of 

 one type in particular, but the determination of a "law" of heredity, 

 a statement of the phenomena of inheritance that will be broad enough 

 to include all types. 



The study of Echinoderm crosses impresses the fact of the actual 

 singleness of type upon one with peculiar force. In the embryos of one 

 cross we may find all three "types" of heredity exhibited, and yet, so 

 far as we can determine, these embryos have been subjected to the 

 same environment. 



How far-reaching this diversity of form may be is impossible to 

 say, for we may make this statement only with respect to the larvae. 

 No one has yet been able to carry hybrid embryos through their meta- 

 morphosis to the adult condition, not to mention the absolutely necessary 

 further step of obtaining individuals of a second generation. This will 

 be a difficult, but probably not an impossible achievement, which will 

 require for its completion the opportunity for working continuously at 

 a marine laboratory for some years. 



SURVEY OF OUR PRESENT KNOWLEDGE OF ECHINODERM CROSSES. 



The primary stimulus to the investigation of Echinoderm crosses 

 was given by Boveri (1889, 1895) in his effort to determine the localiza- 

 tion of the force directing heredity, i.e., to determine whether this force 

 is resident in the nucleus or in the cytoplasm. The Hertwigs had pre- 

 viously seen the entrance of the sperm into enucleated egg fragments 

 and had expressed the opinion that probably nothing would come from 

 such fragments. 



Boveri (1895) made the cross between Echinus microtuberculatus cJ* 

 and Sphcerechinus granularis $ and obtained an intermediate form (text 



