CHROMOSOMES IN ECHINOID EGGS 23 



maternal type simply because the male nucleus has entered too 

 late to take a normal part in the process. A partial union may 

 give rise to a partially thelykaryotic pluteus. Tn some cases an 

 apparently complete union with a subsequent eUmination of 

 Strongylocentrotus chromatin occurred. Baltzer ('10) in his study 

 of the same cross, found that there was no elimination of chro- 

 mosomes under normal conditions, and that plutei with an 

 intermediate type of skeleton were found. 



All that can be claimed for the result of Herbst's treatment 

 is that the sperm was added so late that, when the modified fusion 

 of the pronuclei did take place, the effects of the chemical fertili- 

 zation had attained too great an impetus to be overcome by the 

 materials of the paternal nucleus. He obtained varying degrees 

 of modification. All were not modified ; in all, the result depended 

 on the amount of development that had been attained in the 

 thel^^karyotic activities. 



This is precisely the result that I obtained with the starfish 

 egg ('06) by the superposition of fertilization on artificial partheno- 

 genesis. If the sperm were added before the egg nucleus had 

 gone too far, a union of the pronuclei and a normal division fol- 

 lowed. If the addition were made later, the paternal chromatin 

 entered the mitosis irregularly and was in part rejected. 



Baltzer's important results lie in his accumulation of facts 

 regarding individual chromosomes and in his determination of 

 elimination and non elimination. 



I have shown that his idea of the female Echinoidas the hetero- 

 gametic sex must be restricted to the cases in which it has been 

 observed and cannot be used as a general interpretation. 



I realize the impossibility of interpreting the results of another 

 investigator from drawings, without having seen the sections 

 from which the illustrations were made. It is therefore with 

 hesitation that I suggest that Baltzer's fig. 23a ('10) shows for 

 the Sphaerechinus 9 X Strongylocentrotus 6" cross exactly what 

 I have shown for Toxopneustes 9 X Hipponoe 6". 



Baltzer ('09) has shown that the particular chromosomes of 

 Strongylocentrotus are two long hooks or two long hooks and a 

 short Jiook. (See Baltzer, '09, figs. \a, \h, 5a, 56 and 11, 12 a-d.) 



