MATUIJATIOK OF OVUM IN ECHINUS ESCULENTUS. 203 



This is exactly the mauner in which I have described the 

 short, stout chromosomes of both maturation divisions as 

 opening- out on the spindle. The difference between the two 

 types consists only in the stoutness of the chromatin rods in 

 the polar mitoses, the occurrence of a second longitudinal 

 split in the first division, and consequently tlie absence of 

 the usual longitudinal cleavage in the second division. 



Summary of Results (Text-figs. 2 and 3, pp. 213,214). — 

 The chromatin thread or threads, derived only from a portion 

 of the mass of chromatin in the germinal vesicle, are found 

 split longitudinally and segmented into sixteen bodies — half 

 the number of the chromatin rods in the nuclei of the 

 cleavage divisions. These bodies consist of two short rods 

 placed side by side, and each rod is composed of two spheres 

 united by a less deeply stained portion of the thread. The 

 two rods are intimately associated so as to form a tetrad- 

 like mass, and the whole figure is to be considered a com- 

 pound chromosome. 



After a relatively long prophase each of these is resolved 

 in the first polar metaphase, in such a manner that while the 

 body is opened up along the original cleavage plane, another 

 longitudinal cleft is effected, which is completed in the 

 anaphase, and the final result is another compound chromo- 

 some exactly like the original from which it sprang except in 

 size. Each of the sixteen double rods which remain in the 

 ovum after the extrusion of the first polar body is resolved 

 in the second polar spindle into its two elements without 

 further cleavage taking place. 



In the telophase of the second division the elements Avhich 

 remain in the ovum after the extrusion of the second polar 

 body elongate into rods which become bent on themselves, 

 while those in the second polar body remain condensed as 

 small bilobed rods. 



The maturation phases differ from the ordinary cleavage 

 mitoses in respect of (a) the thickening and condensation of 

 the chromatin rods, {h) the second longitudinal splitting 

 which occurs in the first metakinesis, and (c) the absence of 



