MUSEUM OF COMPARATIVE ZOOLOGY. 623 



omitted), showing male and female pronuclei, in which no nucleolar structures are 

 discernible. An invagination of the yolk into one side of the female pronucleus is 

 compensated by an evagination of the wall of the latter into the substance of the 

 male pronucleus. Compare Fig. 75. Osmic acid preparation. 



Fig, 70. Pronuclei. The position of the internal aster of A^ is indicated by the 

 irregular non-granular area near the female pronucleus, but no radial differentiation 

 can be distinguished. The pronucleoli of about the same number (15 and 16) in each 

 of the pronuclei. Osmic acid preparation. Compare Fig, 72. 



Fig. 70*. Living egg. Recession of the granular yolk from the surface, especially 

 at the primary 'pole, which lies a little to the left of the polar globules. 



Fig. 70''. Early stages in the formation of the pronuclei : a, the female pro- 

 nucleus ; )8, the male. 



Fig. 71. Second polar globule of the egg shown in Fig, 72 with two nuclear 

 structures, as seen after rotation about the primitive diameter as an axis. Osmic 

 acid preparation. 



Fig, 72. Meridional optical section. The pronuclei large, considering the dis- 

 tance between them. The centre of the senescent internal aster of A^ nearly coin- 

 cident with the centre of the female pronucleus, 'as shown by the course of the faint 

 rays still traceable. Osmic acid preparation. See also Fig. 70, 



Fig. 73. Pronuclei still unconsolidated after the appearance of 07ie of the asters of 

 the amphiaster of the first segmentation sphere. Nucleoli numerous. No trace of 

 the complementary half of this amphiaster discoverable. Compare Fig. 80. 



Fig. 74. Both asters of A^ extensively developed ; one distant from the female 

 pronucleus, which still remains unfused with the male pronucleus, although in con- 

 tact with it. 



Fig. 75. A deep cup-shaped invagination of the yolk has forced inward one side 

 of the female pronucleus, and a slight projection from the opposite side of the latter 

 is plunged into the male pronucleus. Both present a wrinkled appearance, but no 

 trace of nucleolar structures. Osmic acid preparation. Compare Fig. 69. 



Fig, 76. Yolk crushed after the formation of the amphiaster of the first segmen- 

 tation sphere. The spindle is proportionately somewhat shorter than before the yolk 

 was crushed. Slender strings of protoplasm stretch from the spindle to one of the 

 fragments of an aster. 



Fig. 77. The two pronuclei near the primary pole, each containing about a dozen 

 nucleoli. Those of poi' are shaded to distinguish them from those of pn. No trace of 

 either aster, Osmic acid preparation. 



Fig. 78. Equatorial optical section of the egg shown in Fig. 58, The outlines 

 of the polar globule and of the two pronuclei "projected," The superficial rays 

 and the granulations of the yolk are omitted, so as to show better the spiral course 

 of the numerous deep rays. Seen from the primary (animal) pole. Consult text, 

 p. 209. 



Fig. 79. Pronuclei seen from the primary pole, each containing about thirty 

 pronucleoli. Only one aster of A^ discernible. Pronuclei not confluent. 



Fig, 80. The egg which is exhibited in Fig. 73, so rotated that the face of the 

 pronucleus nearest the aster appears in profile. The centre of the aster lies at some 

 distance from the sharp outline of the pronucleus. 



Fig. 80*. Egg near the close of the second segmentation. The outlines of two of 

 the blastomeres, and partial outlines of the other pair, as seen from the secondary 



