CHROMOSOMES IN ECHINOID EGGS 19 



In a careful study of fifty-two amphiasters in which all of the 

 elements might be seen with clearness sufficient to enable me 

 to determine the presence or absence of the hook-shaped chromo- 

 some, it was found to be present in twenty-eight instances and 

 not present in twenty-four, from which I have concluded that it 

 occurs in half of the Toxopneustes eggs which have been ferti- 

 lized with Hipponoe sperm. 



The point which I wish to emphasize here is that an unpaired 

 element seen in straight-fertilized Hipponoe eggs is thus shown to 

 he carried by the Hipponoe sperm. 



HIPPONOE 9 X TOXOPNEUSTES d" 



If we turn now to the figures of the reciprocal cross (figs. 12 

 to 20), we will find that in all instances three Vs are present while 

 a hook-shaped chromosome is never found. The identification 

 of the Vs is sometimes made with difficulty since the arms of the 

 V may be brought closely together during anaphase, the result 

 being that the V acquires the appearance of a very thick rod. In 

 figs. 18 and 20 the Vs may be seen with separated arms; in the 

 other figures one or more of the Vs are seen with the arms in con- 

 tact. 



CONSIDERATION OF THE CROSSES 



This cross-fertilization has given us a conclusive means of 

 solving the problem of the origin of the hook-shaped chromosome 

 in Hipponoe. Let me restate our facts.' 



1 . The hook-shaped chromosome is found in half of the straight- 

 fertilized Hipponoe eggs. It is an unpaired element. From a 

 study of these eggs we have no means of determining whether it is 

 an element peculiar to the egg or to the sperm nucleus. 



2. The hook-shaped chromosome is found in one half of the 

 Toxopneustes eggs which have been fertilized by Hipponoe 

 sperm. An element of this form does not occur in Toxopneustes, 

 therefore it has been brought in by the Hipponoe spermatozoan 

 and occurs in half of the spermatozoa. 



3. The hook-shaped chromosome is not present in the Hipponoe 

 9 X Toxopneustes d^ cross. This indicates that it is not present 



