ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 



319 



similar to those of E. ■miliaris ; the hybrid spindles are characterised by having 

 extremely well-developed asters, and the spindle itself is much narrower than that 

 of E. acutiis, and somewhat narrower than that of E. miliaris. This phenomenon 

 may be partly due to the dominant influence of the centrosome introduced by 

 the sperm, and partly due to pathological conditions consequent on cross- 

 fertilization. 



(b) E. miliaris ^ X E. acutus $ (1912). — In accordance with the statement made 

 elsewhere concerning the difiiculty of fertilizing eggs of E. miliaris in 1912 either 

 by their own sperm or by that of other species, the cytology of the eggs showed 

 a very low percentage of mitotic figures. No "vesicles" were ever found in this 

 hybrid, but in the anaphase of the first segmentation division one or moi-e chromo- 

 somes usually either fail to divide or divide so late that they are not included in the 

 daughter nuclei. The condition of these chromosomes is apparently similar to that 

 of the eliminated chromosomes which Baltzer describes in the hybrid Strongylo- 

 centrotus ? X SpluBrechinus $ . 



3. E. esculentus and E. miliaris Hybrids — 



(a) E. esculentus^ X E. miliaris $ (1912). — No elimination of chromosomes was 

 ever observed in this cross. The full number of chromosomes — viz., 36 — is found, 

 this being the number one would expect, as the somatic number of chromosomes in 

 E. esculentus is 38 and in E. miliaris probably 34. The spindles are not so narrow 



Table to illustrate the Nature of the Hybrid Plutei, together with the Cytology of 



the Early Segmentation Divisions. 



