ARTIFICIAL PARTHENOGENESIS AND FERTILISATION. 491 



tissue-cells are to be considered as centrioles, and " nur von 

 den Centriolen nicht aber von den Ceutrosomen, kann dalier 

 gelteU; dass sie allgemeine und dauernde Zellorgane sind." 

 The results of Moi-gan and Wilson can only then be held to 

 prove that centrioles under certain conditions may, by the 

 action of salt solutions, be excited to form centrosomes and 

 radiations round them, for their results might be explained by 

 a multiplication of the two centrioles which the e^g has 

 derived from the last division of the division period, and the 

 distribution of these centrioles through the cell. Even in 

 enucleated fragments there is no proof that the fragment did 

 not contain the centriole of the cell. 



Such a supposition admits of neither proof nor disproof, 

 and the presence of a free '^ centriole" in the unfertilised 

 sea-urchin egg has not been demonstrated. I have seen in 

 young oocytes minute bodies, stained black with iron 

 hasmatoxylin — sometimes double bodies, — but I have not been 

 able to convince myself that they are more than accidents of 

 staining and fixing. 



Turning now to the phenomena of fertilisation in the sea- 

 ui'chin, there is to be recognised (1) a local stimulation at 

 the place of contact of the chosen spermatozoon/ manifested 

 by the streaming out of the protoplasm to form the entrance 

 cone. (2) A general stimulation, manifested by the throw- 

 ing off of the vitelline membrane, and by a change in the 

 constitution of the protoplasm. It becomes more viscid for a 

 time (Morgan) ; a funnel-shaped area of darkly staining sub- 

 stance follows the path of the sperm head (Wilson). (o) 

 A protoplasmic movement focussed on the situation of the 

 middle piece giving rise to the sperm aster. This appears 

 soon after the entrance of the spermatozoon, when the head 

 has begun a movement of rotation. The rotation goes on 



Buller (1902) has sludied tlic question of (lie hearing of clieniotaxis on 

 fertilisation in Eciiinodernis. His conclusion is lliat clieniotaxis plaj's no 

 role in bringing the sexual elements together. The meeting is a matter of 

 chance. The passage through the gelatinous coat is radial in direction, and 

 probalily purely mechanical, thougli possibly due to stereotaxis. 



