MATUEATION OF OVUM IN ECHINUS ESCULhJNTUS. 199 



eighteen in the spermatocytes, and eight or nine in the 

 spermatids. He confesses to great uncertainty in regard to 

 these figures on account of the minuteness of the chromo- 

 somes, and the hist figure is quite out of harmony both with 

 R. Hertwig's counts and my own. 



Careful analysis of this tetrad body shows that it is com- 

 posed of two short stout rods placed side by side (figs. 8 

 — 10, 12, and 23). The ends have the form of little spheres, 

 and looking back to fig. 14 one may conclude that they corre- 

 spond to the spheres seen in that thread united in pairs, but 

 there is no transverse cleavage of the thread between the 

 four spheres. A complete tetrad, consisting of four indepen- 

 dent round bodies as figured for Ascaris, or the mole-cricket, 

 does not occur in Echinus. Further, one cannot regard the 

 two rods as separate and independent at this stage ; they 

 are bound together closely, and the figure is really a com- 

 pound chromosome. 



According to the above interpretation the tetrads thus arise 

 by a single longitudinal split of an original thread or threads. 

 At no time are there any ring or other irregular figures, as 

 described in so many other cases. The possibility, is not 

 excluded, that the groups might result from conjugation of 

 the dyadal bodies in pairs, as described by Wilcox (1895) 

 in the grasshopper, and Calkins (1895) in the earthworm. 

 In two instances only out of a large number of prophase 

 stages have I seen a figure other than those described. In 

 one section, just before the spindle is formed (fig, 15) there is 

 a double comma form, which appears in all other sections at 

 a later stage. 



As the compound chromosomes are gathered into the 

 equatorial plate they lie irregularly, and in the metakinesis 

 they do not seem to be resolved simultaneously, for in all my 

 sections of this stage, about sixty in number, figures in 

 different phases are seen, and as the chromosomes lie 

 throughout the whole equatorial plate, and not only round 

 the periphery of the spindle, various irregular bodies are 

 seen which are portions only ot whole chromosomes. The 



