194 THOMAS H. BETCE. 



tendency to the formation of multiple centimes of activit}', or 

 putting it in terms of the centi^osome, to the formation of four 

 centrosomes instead of two. 



The whole process leading up to the formation of the first 

 polar amphiaster is very complicated in Echinus, and 

 extremely difficult to trace in sections. It is as difficult 

 to be sure of the phases in the living Echinus e^g, which 

 are moreover difficult to get as the process normally 

 takes place within the ovary. The process does not 

 seem to me to be so simple as it has been described for 

 Asterias ; indeed, it is in many respects like what Mead has 

 described for Chfetopterus. Dr. Teacher has recently studied 

 the phases in the living egg, and has kindly let me see his 

 results, which supplement my own. He has seen frequently 

 a stage which I have described as follows — " near the surface 

 of the ovum is a clear granular area having the appearance 

 of ground glass, surrounded by a darker ring merging into 

 the alveolar-looking cytoplasm. This ring, at its circumfer- 

 ence, is distinctly irregular, and suggests delicate radiations 

 from the central granular area." This is obviously the 

 stage represented by the section depicted in fig. 9, and 

 corresponds to the transformed germinal vesicle. At this 

 stage I could not make out distinct asters in the living egg, 

 and in the sections the astral rays which lie within this area 

 are of great delicacy. Dr. Teacher has seen in this phase 

 many specimens with a number of asters in the cytoplasm 

 around the transformed germinal vesicle, and has made out 

 at the same time, within the area itself, astral formations 

 which he believed to be the definitive asters of the spindle. 

 While, therefore, I have in the foregoing description traced 

 the centrosomes as if they were persistent centres travelling 

 through the cytoplasm, I cannot exclude the possibility of 

 their free formation as described by Mead. 



Fig. 16 represents the spindle now completed. Tiie 

 chromosomes are being drawn into the equatorial plate. 

 Fig. 17 shows the now completed spindle in nu;taphase. 

 ft is relatively bulky, with blunt and rounded ends, and 



