MATURATION OF OVUM IN ECHINUS ESCULKNTUS. 223 



PLATE 10. 



Fig. 1. — Early oocyte. 



Fig. 2. — A stage in the growtli of tlie genniiial vesicle, showing also the 

 appearance of the cytoplasm when the preparation is not strongly washed out. 



Fig. 3. — Another phase in the growth of the germinal vesicle, showing also 

 tiie appearance of the cytoplasm in a preparation from which the iron hsema- 

 toxylin Las been strongly washed out. 



Fig. 4. — Section through invagination into germinal vesicle. 



Fig. 5. — Disappearance of nuclear membrane ; rejection of greater part of 

 chromatin network; isolated mass of chromatin from which chromosomes are 

 derived. 



Fig. 6. — Later stage, in which two asters have appeared. 



Figs. 7 a and T b. — Two adjoining sections througii the same germinal 

 vesicle, showing the mass of chromal.in from which chromosomes will be 

 formed. 



Fig. 8. — Tiie two definitive asters ; chromosomes in form of bilobed bodies 

 or tetrads. 



Fig. 9. — Another section, showing the chromosomes within a central plate, 

 surrounded by a crown of radiations fading imperceptibly into the cytoplasm ; 

 one aster in view, a second lay in tiie adjoining section. Sometime s a third 

 aster is seen at this stage. 



PLATE n. 



Figs. 10 — 11 represent the dilferent forms assumed in various sections 

 by the chromosomes before tlie formation of the first spindle. The dotted lines 

 indicate the outline of the central plate. 



Fig. 15. — Asters now in radial position; early phase of first division 

 spindle. 



Figs. 16, 17. — Two phases of metaphase of first division. 



Fig. 18. — Same stage semi-diagrammatic. The various metaphase figures 

 in two adjoining sections of the same ovum are each carefully drawn, but 

 represented in the same plane. 



Kig. 19. — Anaphase of first maturation division. 



Fig. 20. — Slightly oblique section of constricted spindle in formation of 

 first, polar body. 



Fig. 21. — Various chromosomal figures seen during progress of first division, 

 in profile and face views. 



Fig. 22. — The ehromosonuis n^taincd in llii' ovum after the extension of 

 the first polar body, as seen in an oblique section of central aster. 



