March 10, 1005.] 



SCIENCE. 



379 



stimulate dormant potentialities into ac- 

 tivity, or to bring together the isolated 

 elements of a character which in its com- 

 plex form has been lost. 



Artificial Parthenogenesis in Thalassema 

 mellita: George Lefevre, University of 

 I\Iissouri. 



An investigation of artificial partheno- 

 genesis in Thalassema mellita has shown 

 that the eggs of this worm can be induced 

 to develop into actively swimming trocho- 

 phores, in the absence of sperm, by immer- 

 sion for a few minutes in very dilute solu- 

 tions of acids both inorganic and organic. 

 Nitric hydrochloric, sulphuric, carbonic, 

 acetic and oxalic acids were used success- 

 fully, and in favorable experiments 50-60 

 per cent, of the eggs developed into swim- 

 ming larvK that could scarcely be distin- 

 guished from normal trochophores of a 

 corresponding stage. 



An egg membrane invariably forms 

 shortly after removal from the acid solu- 

 tions, and maturation, identical with the 

 normal process, frequently occurs. In a 

 m;mber of cases, however, polar bodies were 

 not extruded, but the eggs divided and 

 eventually gave rise to trochophores with- 

 out any external indication of maturation. 

 On sectioning such eggs it has been deter- 

 mined that the maturation process occurs 

 internally, the polar mitoses taking ])Iace 

 below the surface and without accompany- 

 ing cytoplasmic division. The result is 

 that, in some cases at least, four resting 

 nuclei are formed in the cytoplasm of the 

 egg which represent the egg-nucleus and 

 the nuclei of the three polar bodies. These 

 four nuclei then come together and fuse to 

 form a cleavage-nucleus, which, therefore, 

 contains, in addition to the chromatin of 

 the egg-nucleus, all the chromatin that 

 would have passed into the polar bodies, 

 had they been extruded. 



The egg-centrosome disappears after the 



formation of the second polar body, and 

 the cleavage-centrosomes arise de novo. 



It has frequently been observed that the 

 polar bodies continue to divide, with the 

 result that they form a morula-like cluster 

 of minute cells. 



Cell-divisions take place mitotically, and 

 in many cases the early cleavage is per- 

 fectly normal, although a great variety of 

 abnormal cleavages also occur. 



The larvae arising parthenogenetically are 

 strikingly normal in appearance and struc- 

 ture and exhibit the usual differentiations 

 characteristic of the normial larviE of a cor- 

 responding stage of development, digestive 

 tract and mouth, prototrochal band, apical 

 plate and flagella, etc. 



Further Experiments on Self and Cross 

 Fertilization in Ciona: T. H. ]Morgan, 

 Columbia University. No abstract. ■ 



A Few Words on ^Yhat is to he Understood 

 hij 'Good' Fixation: Alexander Pe- 

 TRUNKEViTCH, Harvard Uiiiversity. 

 The question of what is to be understood 

 by 'good' fixation is of both theoretical and 

 practical value. Authors disagree as to 

 how much of what we observe under the 

 microscope is artefact and consequently as 

 to the trustworthiness of the conclusion 

 drawn from it. In my opinion, as fixed 

 matei-ial consists in artefacts only, we 

 should learn to eliminate errors by com- 

 parison. Two errors ax'e especially to be 

 shunned: mistaking for true, (1) struc- 

 tures which result from a dislocation of 

 cell-organs, (2) those created by the use of 

 injurious agents. Pauli placed the alveolar 

 structure of protoplasm in the latter group, 

 demonstrating that colloids show no separa- 

 tion into two 'phases' when a normal solu- 

 tion of urea is added to the fixing liquid. 

 I repeated his experiments with a great 

 variety of agents, some of them never used 

 before. The results are the opposite of 

 those obtained by Pauli. 



