The maturation and fertilization of the egg of Cerebratulus. 475 



Fig. 38. The asters are still increasing in size. Those fibres 

 directed towards opposite halves of the cell, instead of crossing as 

 formerly, bend outwards where they come in contact at the equator. 

 The two centrosomes in each of the greatly enlarged centrospheres 

 have separated and passed towards its outer border. Each centrosome 

 has increased slightly in size, and is imbedded in a homogeneous matrix 

 from which a few delicate radiations extend into the substance of the 

 surrounding centrosphere. These are shown in Fig. 38 a, which also 

 shows the vesicular or reticular character of the centrosphere. Fig. 38 a 

 is enlarged about 1500 diameters. 



Fig. 39. The early telophase. The aster-fibres have elongated 

 until they reach nearly to the periphery of the egg. The centrospheres 

 are greatly swollen, and no longer possess a definite outline. The pair 

 of minute asters , which were noticed in Fig. 38, have become much 

 more evident, and have migrated outward nearly to the periphery of 

 the centrospheres. They are, in consequence, widely separated from 

 the chromosomal vesicles. 



Fig. 40. The first division is just completed. The enormous centro- 

 spheres which characterized the preceding stage have disappeared 

 entirely. The old radiations may still be followed nearly to the peri- 

 phery of the cell, but have become irregular and much less distinct. 

 The new asters are increasing rapidly in size. Their radiations are 

 sharply defined, though delicate, and extend among the inner portions 

 of the old rays. Each nucleus has a slight indentation on the side 

 next the asters. Reconstructed. 



Micrura caeca. Spermatozoon just within the egg. 



X 



C. marginatus. Spermatozoon with young aster. X 750. 

 The sperm-centrosome has divided, but not the aster. 



X 



The sperm-centrosomes are separating with the formation 

 of a delicate central spindle. X '^^0- 



Fig. 45. The sperm-asters are dividing. A beautiful central spindle 

 is seen between the centrosomes. The rays from the two asters cross 

 each other. Between the asters, and running at right angles to a line 

 joining them, are seen the degenerating fibres of the middle portion of 

 the original aster. X 750. 



Fig. 46. The central spindle between the two sperm-asters is 

 disappearing, while above and below it are seen the degenerating fibres 

 of the original sperm-aster. The spermatozoon is being left behind. 

 X 750. 



Fig. 47. The sperm-asters have separated almost completely. 

 X 750. 



Fig. 48. Micrura caeca. Transverse section of a first polar spindle 

 before the division of its sixteen chromosomes. X 1000. 



Fig. 49. M. caeca. The formation of the first polar body. 

 X 1000. 



