No. 3.] STUDIES OiV CEPHALOPODS. 255 



qu'il avait dans ranimal enticr. C'est quelque chose de com- 

 parable a Texperience de I'aimant brise. Ne peut-on en conclure 

 que chaque Element histologique possede, lui aussi, ces deux 

 polarites de I'animal, polarites qui persistraient dans la cellule- 

 oeuf, apres qu'elle a cesse de faire partie des tissus maternels?" 



Mark ^ has recently called attention to the fact that in the 

 ovariaii ov?tin of Lepidosteus, the polar differentiation of the 

 ovum can already be pointed out by the existence of a peculiar 

 secretory activity of the egg-protoplasm at one point, which cor- 

 responds to the future micropyle region. According to Mark, 

 the egg-membranes of the ovum — the villous layer and the zona 

 radiata — are produced by the secretion from the surface of the 

 ovum, and the change in the egg-membranes corresponding to 

 the region of the future micropyle is due to the diminished se- 

 cretory activity of that particular part of the ovum. 



Mention must be made in this connection of the important 

 fact established by Rabl,^ Carnoy,^ and Gehuchten,* that the 

 resting nuclei in the tissue-cells of certain animals, such as the 

 epithelial cells of the Salamander, the testis cells of a certain 

 Arachnid and of Crustacea, the intestinal cells of a larval 

 Dipteron, there are either two poles or an axis, around which 

 chromosomes arrange themselves in a definite way. 



The preceding, taken somewhat at random, will show one 

 essential fact ; viz. that, however diverse the examples, they all 

 point to one and the same conclusion, namely, that the metazoan 

 ovum and its derivatives, the tissue-cells, are more than a homo- 

 geneous, isotropic mass of protoplasm, devoid of definite sym- 

 metry. The study of caryokinetic figure shows, van Beneden 

 points out, that the cell is not only uniaxial, but also bilateral. 

 In several forms of ova carefully studied, the axes of the caryo- 

 kinetic figure correspond in a definite way with the recognizable 

 axes of a given ovum, the external shape of which is chiefly 

 determined by the quantity and distribution of the food-yolk. 

 The axes thus determined are maintained through the differ- 



1 E. L. Mark : Studies on Lepidosteus. Bull. Mus. Comp Zool. Harvard College. 

 Vol. XIX, No. I, 1890. 



2Rabl: Uebey Zelltheilung, Morph. Jahrb., Bd. X, 1S85; Ueber Zelltheilung, 

 Anat. Anzeiger, IV, Jahrg. No. I, 1889. 



^ Carnoy: La Cytodierhe chez la Art/tropodes. La Cellule, Tome I, 1885. 



*Gehuchten: L' Axe organique du noyau. La Cellule, Tome V, 1889. 



