Hybridization of Echinoids. 23 



another, and feel that the facts confirm the statement I have made. 

 As to the nature of these granules, two possible explanations suggest 

 themselves: Either they may be bodies containing material which 

 enters into the cortical reaction alone in fertilization, in which case it 

 would seem that there must have been a more complete reaction in 

 the cross-activation than in direct activation, or (and this seems the 

 more probable explanation) they contain the sperm-agglutinating 

 substance described by Lillie, and a large part of this has been given 

 up by the eggs during their two hours staling in sea-water before 

 insemination. It should be a matter of little difficulty, if suitable 

 material were available, to test the latter explanation by determining 

 whether there is a progressive diminution in number of superficial 

 granules in unfertilized eggs allowed to stand for some hours in 

 sea-water. A positive proof of this point would not preclude the 

 idea that their substance is also concerned in the reaction of the egg 

 to the spermatozoon. 



Study of Chromosomes in Species-Fertilized Cidaris Eggs. 



The study of chromosomes both in the straight-fertilized and in 

 the cross-activated eggs has been prolonged and has been based 

 upon an abundance of material. Some evidence which has been of 

 value has been obtained by the study of eggs in which development 

 was initiated by Loeb's butyric-acid method. 



The chromosomes in the straight-fertilized Cidaris eggs are crowded 

 closely together; they are sip.all and difficult to count. My study 

 of the morphology of the chromosome groups has been mainly of 

 sections showing polar views or of lateral views of the division figure 

 in anaphase. In both types of section it is usually very difficult to 

 distinguish between a fragment of a chromosome and an entire 

 chromosome. In sections 5 microns in thickness, which pass through 

 the spindle in the direction of its long axis, chromosomes will be 

 found usually in three successive sections. Should the plane of 

 the section be slightly oblique to the long axis of the spindle, there 

 may still be chromosomes in three successive sections of the egg, but 

 there will be only two sections of each anaphase plate. When the 

 sections have been of this type I have combined the three sections in 

 two figures, as in figures 7a and 76, the latter containing the chromo- 

 somes from the first and third sections of the series. In such figures 

 as this, therefore, sister chromosomes do not stand opposite each 

 other in the anaphase plates as shown. The danger of overcounting 

 the number of chromosomes in the longitudinal sections of the 

 division figures because of the sectioning of individual chromosomes 

 is probably offset by the fact that some of the chromosomes lie 

 directly under one another and on this account may be overlooked, 

 even in the most careful focusing. 



