134 Papers from the Marine Biological Laboratory at Tortugas. 



(2) There is a difference in intensity of displacement corresponding 

 with the stage at which fertilization takes place; the maximum dis- 

 placement coinciding with the greatest expansion of the egg nucleus 

 before its solution. After the passing of the maximum size there is a 

 gradual falling of displacement, but not to zero. 



(3) Larvae with maternally directed heredity, as a rule, have larger 

 nuclei than the ordinary hybrids of the same culture. 



(4) The hybrids with nuclei larger than the normal may have 

 arisen by copulation of a diplothelykaryon and an arrhenokaryon. 



(5) Partially-thelykaryotic larvas may arise when the sperm nucleus 

 copulates with one of the daughter nuclei succeeding dyaster formation. 



(6) Hybrids of the maternal type with small nuclei may have 

 arisen because of the elimination of male nuclear material. 



(7) Hybrids of the paternal type with small nuclei are probably 

 arrhenokaryotic, the maternal nuclear material having been eliminated. 



(8) Hybrids may be partially arrhenokaryotic in character. 



(9) Displacement to the maternal side may be caused in various 

 ways. 



(10) The course of heredity may depend on relative proportion of 

 female nuclear mass. 



NEW INVESTIGATIONS. 



There is, as is well known, a characteristic period of time between 

 fertilization and the appearance of the first cleavage furrow. I bring 

 these statistics together in this place for the sake of comparison. Fertil- 

 izations were made at temperatures varying between 26 and 31 C. 

 Arbacia, 40 to 47 minutes; Hipponoc, 60 to 75 minutes; Mellita, 30 to 60 

 minutes, usually 35 minutes; Moira, 35 minutes; Toxopneustes, 37 to 

 45 minutes. These variations are not altogether correlated with differ- 

 ences in temperature. 



DESCRIPTION OF NORMAL PLUTEI. 



One is much impressed by the more rapid earlier development of 

 the forms considered in this paper than of the forms described by Euro- 

 pean investigators. The following outline of the normal development 

 of one series of Toxopneustes eggs may be of interest. 



(Fertilization made at i2 h 4o m p. m. Temperature of water, 28 C.) 



First cleavage ih 20 m meridional 2 cells. 



Second cleavage ih=;o m meridional 4 cells. 



Third cleavage 2 h i5 m horizontal 8 cells. 



Fourth cleavage 2 h 30 m horizontal (micromeres) 12 cells. 



Fifth cleavage 2^30 meridional of upper 4 16 cells. 



Sixth cleavage 2h5om meridional of lower 4 20 cells. 



Seventh cleavage 2 h 5o m meridional of upper 8 28 cells. 



Eighth cleavage 3h om horizontal (mic. to form second set) ... 32 cells. 



Ninth cleavage 3 h i5 ra horizontal (div. of lower 8) 4 cells. 



Tenth cleavage 3 h 35 m meridional (up. 16 to 32) 56 cells. 



Eleventh cleavage 3 h 35 m meridional (next 8 to 16) 64 cells. 



Twelfth cleavage 3 h 45 m meridional (first mic. to 8) 68 cells. 



Thirteenth cleavage 3 h som meridional (lower 8 to 16) 7 6 cells - 



