ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 323 



Summm^y of Cytological Section. 



1. In all the crosses there is a true fusion of the male and female chromatic 

 substance at fertilization. 



2. In a number of the crosses a few chromosomes are definitely eliminated in the 

 first and second segmentation divisions. 



3. In the cross E. esculentus $ X E. acittus $ no elimination takes place, while in 

 the reciprocal E. acutus $ X E. esculenhis $ an elimination of a few chromosomes 

 invariably takes place. This elimination, however, cannot be in this case correlated 

 with any feature in the inheritance, on account of our knowing no definite distinction 

 between the larvae of these two forms, which we might investigate from this point 

 of view. 



4. In the cross E. esculentus ^ X E. miliaris $ no elimination takes place, while 

 in the cross E. acutus % X E. milians $ , again, a constant elimination of a few 

 chromosomes always takes place. These two crosses have always resulted in maternal 

 inheritance, despite the fact that elimination only takes place in one, i.e. E. acutus ? 

 X E. miliaris $ . In this cross we have a definite feature which we can 

 investigate. 



5. In the crosses E. miliaris ^ X E. esculentus $ and E. miliaris ? X E. acutus <? , 

 whose inheritance was maternal in 1909-11 and paternal in 1912, material only of 

 1912 was examined, so that it is impossible to say if the difierence of inheritance 

 between 1909-11 and 1912 was correlated with differences of elimination. 



6. The elimination of chromosomes is connected with the formation of chromatin 

 vesicles. 



14. Sea Temperature as Affecting Inheritance, etc. 



In a previous section we have stated that in 1912 the inheritance in hybrids made 

 with E. 'miliaris as the maternal parent was different from what it had been in 

 former years. That this changed inheritance was due to some factor operating on 

 the germ cells before fertilization is indicated by various facts which are fully stated 

 in Section 16 (see p. 342). The conclusion was forced upon us that some environ- 

 mental condition had so changed the female germ cells of E. miliaris that they no 

 longer transmitted the characteristics of the species to hybrid ofispring. 



In this section we suggest that there mcty be a connection between the changes in 

 temperature of the inshore waters, during the winters preceding the experiments, 

 and the types of inheritance found in the hybrids. A set of curves is given showing 

 the temperature of the sea-water at Plymouth for the years embraced by our 

 experiments. They cover each year the period for Echinus during which the germ 

 cells are undergoing growth and maturation. They have been constructed from 

 weekly means, furnished us by the Meteorological Oflice, obtained from temperatures 

 taken daily from the end of the Promenade Pier, Plymouth. They correspond but 



2 T 2 



