390 Louise Hoyt Gregory 



separating the endoplasm from the cortical layer, there is a thick- 

 ening of the protoplasm that marks the limits of each layer. 



As in the majority of infusoria, the nuclear material is different- 

 iated into two structures, a large macro-nucleus, and one or many 

 small micro-nuclei. The macro-nucleus lies dorsal to the pharynx 

 in the anterior half of the body, near the middle line, not in the 

 extreme anterior region as is stated in Gruber's account. Neither 

 is the early statement true, that the nucleus is visible only in young 

 forms in which the protoplasm is less dense. I have been able 

 to see the macro-nucleus at all stages of growth, in the large ma- 

 ture forms as well as in the small young individuals. In the fixed 

 material the nucleus is always visible, staining with different de- 

 grees of intensity, and surrounded by a definite membrane. The 

 question of a nuclear membrane has long been a subject of dis- 

 cussion. Many investigators, among whom are Biitschli ('98), 

 and Albrecht ('03), assert that no membrane exists. Biitschli does 

 not consider the membrane of an infusorian a true one, since it is 

 transitory, and may have the same reactions as the cytoplasm. 

 Albrecht, experimenting with sea urchin eggs, found that if the 

 nuclei were forced from the egg by compression, and brought in 

 contact with another, they would flow together, hence he believes 

 that there is no nuclear membrane. Marcus ('07) repeated Al- 

 brecht's experiments, using Actinosphjerium, and obtained oppo- 

 site results; that is, the nuclei did not flow together. Albrecht 

 probably broke the membrane when compressing the eggs. I 

 have compressed Tillina, and both the macro- and the micro- 

 nuclei were forced from the body. They always retained their 

 normal shape, and showed the presence of a membrane. This, 

 however, may vary at times in its definiteness, and in two 

 cases seemed to have been broken, allowing the nuclear fluid and 

 cytoplasm to mingle. 



The shape of the macro-nucleus varies. It is usually an ellip- 

 soidal body, the long axis measuring 50// to 70//, the short axis 

 20// to 30//. At other times the shape may be spherical or like 

 the letter U (Text Fig. 2, a, b). The different forms may all be 

 derived from the normal ellipsoidal one, and may represent a 

 certain stage in the preparation for division. The macro-nucleus 



