No. 2.] THE SEA-URCHIN EGG. 467 



hardly possible to doubt its truth. Whether it represents the 

 whole truth is, however, far from certain, and it must be 

 remembered that Strasburger asserts in the most positive 

 manner^ that the hypothesis is absolutely untenable as an 

 explanation of the movements of the chromosomes in plants. 



A study of the facts in Toxopncjtstes shows, I think, that as 

 far as the divergence of the daughter-chromosomes is con- 

 cerned, it is equally untenable in this case. As shown in 

 Text-fig. VIII, and in Phototype 8, the spindle is barrel-shaped, 

 its truncated ends forming the boundary of the centrosphere at 

 one side, where the spindle-fibres abruptly stop. The chromo- 

 somes proceed to the extre^ne limit of the spiiidle and lie in con- 

 tact with the centrosphere. This would be impossible were 

 their movements caused by the contraction of fibres stretching 

 between them and the centrosphere. This case shows con- 

 clusively, that while the contractility of spindle-fibres may be a 

 true explanation of the chromosomal movements in some cases, 

 yet it cannot be regarded as a universal or the only cause ; and 

 in the present fragmentary state of our knowledge it must 

 remain for future investigation to refer these movements to a 

 common agency, 



V. General Summary of Observations. 



A. The Archoplasm. — i. The sperm-aster arises by the 

 morphological rearrangement of the general cyto-reticulum 

 under the influence of a central mass derived from the middle- 

 piece of the spermatozoon, 



2. The astral rays arise by the linear arrangement and 

 fusion or close union of the granules or microsomes of the 

 reticulum. 



3. The spindle-fibres are entirely formed within the nucleus, 

 being progressively differentiated out of the linin network from 

 the centers of the asters, precisely as the astral rays arise from 

 the cyto-reticulum. 



4. At the close of karyokinesis the spindle-fibres break up 

 into granules (which may be loosely aggregated to form a tran- 



1 Aftai. Anz., VIII, 67, 1893. 



