70 Papers from the Marine Biological Laboratory at Tortugas. 



FIG. 37. Oocyte showing the passage of chromatin from nucleolus to adjacent chromo- 

 some group just prior to maturation. < 700. 



FIG. 38. Nucleus in which the chromosome mass and nucleolus are at opposite poles. 

 The chromosomes are in position to meet the polar spindle, which is about 

 to form. The chromatin of the nucleolus is seen leaving the vacuolated 

 plastin ground-substance (plasmosome) en masse and moving toward the 

 chromosomes, at the same time contributing chromatin to the achromatic 

 nuclear network. The peripheral layer of yolk granules is also shown. 

 X 1320. 



FIG. 39. Primary oocyte at culmination of growth-period and some time prior to 

 maturation. The eccentric nucleolus is homogeneous, with some of the 

 chromosomes closely attached. The nuclear network is wide-meshed, 

 beaded, and slightly chromatic, and with occasional karyosomes. The 

 peripheral layer of the egg is rilled with large yolk-granules. X 1320. 



PLATE 3. 



FIG. 40. Nucleus with chromosomes gathered into three groups (a, b, and c). These 

 are enlarged at a, b, and c, X 2100. The majority of the chromosomes 

 are bilobed. Their number is approximately 18. X ?oo. 



FIG. 41. Oocyte in which the chromosome mass lies very close to the nucleolus. X 44O. 



FIG. 42. Oocyte showing the beginning of the establishment of a chromatin connec- 

 tion between nucleolus and chromosome group. X 440. 



FIG. 43. Oocyte in which the chromosome mass is less compact. Nucleolus is break- 

 ing up and establishing connection with the chromosomes. X 700. 



FIG. 44. The above mass of chromosomes magnified X 2100. About 15 chromosomes 

 may be counted. 



FIG. 45. Cross-section of central pole of first maturation spindle after first polar body 

 has been constricted off (this seen in next section), showing 20 chromo- 

 somes, the excess of 2 chromosomes above the usual number of 18, 

 due to a premature longitudinal splitting of two of the elements. Com- 

 pare with figure 44. X 2100. 



FIG. 46. Maturing oocyte in which spindle is forming and the astral rays have pene- 

 trated far into the nucleus. Both chromosome group and fragmenting 

 nucleolus near distal pole of nucleus. There are no chromosomes among 

 the rays in this or any of the adjacent sections. X 700. 



FIG. 47. Oocyte in initial stages of maturation, showing the nucleolus in transit toward 

 the polar spindle, leaving a vacuole (the remains of the resorbed plastin 

 ground-substance) and chromatin particles behind. The chromosome 

 group has already separated and some of its elements have passed among 

 the astral rays. X 700. 



FIG. 48. Composite figure of two consecutive sections. Nucleolus seen to break up 

 at one pole of the nucleus, leaving a plastin remnant. Spindle forming 

 at opposite pole with several chromosomes already drawn toward it. 

 X?oo. 



PLATE 4. 



FIG. 49. Primary oocyte showing nucleolus and chromosomes at opposite poles of 

 nucleus. Maturation is imminent and the chromosomes are in proper 

 position to be drawn into the spindle soon to appear. Chromatin in 

 form of beaded threads is seen to pass from nucleolus toward chromo- 

 somes. X 700. 



FIG. 50. Two centrospheres with centrosome and aster pushing into nucleus. Nuclear 

 wall still intact. Nucleolus in process of dissolution. Two chromosomes 

 are seen emerging from the mass. X 1320. 



FIG. 51. Composite figure of three consecutive sections, showing one aster (a), a 

 group of chromosomes scattering (several are detaching themselves from 

 nucleolar mass of chromatin), and a spherical chromatin mass (b~), and 

 two additional spherical nucleolar masses (c). X 700. 



FIG. 52. Chromosomes are entering the polar spindle. Last one in process of being 

 detached from mass of disorganizing nucleolus. Nuclear network very 

 close-meshed and highly chromatic. X 1320. 



FIG. 53. Nuclear wall is ruptured. First polar spindle is being formed. Nucleolus 

 is breaking up. Chromosomes are becoming attached to astral rays. Nu- 

 clear network is becoming close-meshed and more chromatic. X 1320. 



