CROSS FERTILIZATION. 175 



The majority of the enucleated eggs were about three fourths 

 the size of a whole one. None were less than one half size and 

 many were nine tenths size or even larger since every effort was 

 made to remove the nucleus with a minimum amount of cyto- 

 plasm. Within these size limits, from one half to nine tenths, 

 there is no correlation between the size and the capacity to 

 respond to activation by Arbacia sperm. This indicates that 

 failure to secure development in such experiments is not due to 

 the size of the fragments if they be at least half the size of a 

 whole egg. The lowest size limit capable of activation was not 

 ascertained. 



Just ('19) found that the best method of activating Echi- 

 narachnius eggs with Arbacia sperm is by the use of a heavy 

 sperm suspension. In the present experiment with enucleated 

 eggs various dilutions of sperm suspension yielded about the 

 same percentages of activation, so long as the suspension was at 

 least three drops of seminal fluid to 50 cc. of sea water. Heavier 

 suspensions do not modify the amount of development. 



Whether the enucleated eggs are activated a few minutes 

 after cutting, or as late as an hour afterwards, the percentage of 

 development is the same. A period of recovery from so-called 

 "shock" is not required. 



The 689 enucleated eggs yielded forty six blastulae, giving 

 about six per cent, development. Whole Echinarachnius eggs 

 when similarly activated with Arbacia sperm yield an average of 

 eighteen per cent, development (Just, '19). This difference in 

 percentage of development is probably a result of the general 

 maltreatment of enucleation. 



It would be expected that there might be a considerable 

 amount of irregular development among enucleated eggs caused 

 by polyspermy. In most cases, however, either the enucleated 

 eggs cleave regularly or they do not cleave at all, and there is 

 little irregular cleavage. Although polyspermy and consequent 

 irregular cleavage occur in fragments obtained by shaking eggs 

 to pieces (Morgan, '96), polyspermy, as judged by irregular 

 cleavage, is rare in fragments obtained by cutting with a glass 

 needle. The shaking method is probably quite harmful to the 

 egg and wrecks its mechanism for controlling polyspermy, 

 whereas the glass hair leaves it in a more normal condition. 



