No. 542] CHROMOSOMES IN ECHINOID LARVAE 



73 



with a Hippono'e skeleton. The largest percentage of 

 the eggs does respond in this manner; a smaller per- 

 centage dies in the blastula and gastrula stage, and a 

 still smaller percentage shows little or no trace of pater- 

 nal influence. 



. If we take the eggs of such a fertilization and examine 

 them during the segmentation stages, we shall see that in 

 these eggs the greatest number show normal division 

 figures, and by normal I mean those of the almost dia- 

 grammatic sort which may be seen in straight-fertilized 

 Toxopneustes eggs, figures which give no indication of 

 any cnromosome elimination, while the smaller number 

 show aberrant figures with varying degrees of elimi- 

 nation. 



The normal division figures correspond in number to 

 the plutei with the paternal type of skeleton and the 

 aberrant figures to the plutei of intermediate and mater- 

 nal character. 



Clearly, then, in the hybrids of Echinoid crosses we 

 may have dominance of one species or the other with 

 respect to the skeleton, and this dominance may be trans- 

 mitted by the egg or by the spermatozoan. Of the con- 

 siderable number of crosses that I have made, only one, 

 the Hippono'e <$ X Toxopneustes 2 t cross gives this 

 evidence. 



Some facts of interest are demonstrated by the Arba- 

 cia X Toxopneustes crosses. These crosses- are made 

 with difficulty and I had never succeeded in getting them 

 until the past summer. The chromosomes in Arbacia 

 are much smaller than those in Toxopneustes and I had 

 hoped that material from this cross would be of use when 

 compared with my experimental Toxopneustes material. 

 Here, however, I found that there is an elimination of 

 chromosomes from the first, an elimination which may 

 involve the rejection of not only the foreign chromo- 

 somes, but some of those of the egg as well, the result 

 being that, in some instances, the full haploid number of 

 neither is retained, and few zygotes pass through the 

 blastula stage. 



