ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 575 



use of the " outer layer " of the segmented ovum, Prof. A. C. Haddon 

 suggests an explanation of these facts. 



The " outer layer " corresponds to the non-emhryonic epiblast of 

 the area opaca : the middle layer of the blastoderm is the embryonic 

 epiblast, aud the deep, flat cells, form the hypoblast. The " blasto- 

 pore " of Van Beneden indicates in an exaggerated manner the 

 separation between the embryonic and non-embryonic germinal layers, 

 since the blastoderm has sunk into the blastodermic vesicle owing 

 to the absence of yolk. 



The author gives a series of woodcuts of hypothetical mammalian 

 eggs, in which is shown the manner in which the true embryonic 

 epiblast (which lies at first, as in fowl's egg, on the surface of the 

 yolk) sinks into the yolkless vesicle ; the non-embryonic epiblast, 

 which has now extended round the blastodermic vesicle, owing to the 

 loss of yolk, gradually grows over the in-sunk embryonic epiblast : 

 the stage before the meeting of the sides of the embryonic epi blast 

 being represented by the stage in the actual mammalian egg when 

 Van Beneden's " blastopore " is present. The cells of the embryonic 

 epiblast now arrange themselves in a definite layer below the non- 

 embryonic epiblast or covering cells, and below it again is the 

 hypoblast, as in the actual mammalian blastodermic vesicle. The 

 segmentation of the mammal's ovum is very abbreviated ; the first 

 cleavage furrow demarcates the embryo from the yolk-sac. 



The author then refers to the researches of Agassiz and Whitman 

 and others as to the orientation of the primitive segmentation spheres. 



In the marsupials it has been shown that the subzonal membrane 

 of the yolk-sac serves to attach the embryo to the wall of the uterus, 

 either by vascular villi or by simple amceboid processes of the cells : 

 so in the rabbit the covering cells, or non-embryonic epiblast of the 

 blastodermic vesicle (i. e. yolk-sac) " form the first adhesion between 

 the ovum and the parent." 



This temporary adhesion in the Eutheria is later on replaced by 

 allantoic villi. 



Horny Investments of the Egg's of Scyllium stellare.* — Herr 

 C. F. W. Krukenberg gives a full account of his experiments on the 

 egg-shells of Scyllium stellare, and points out that in some the 

 substance resembles cow's-horn and human hair. 



£. Histology, f 



Phenomena of the Division of the Cell-nucleus.:}: — M. L. Guignard 

 directs attention to some of the phenomena which accompany the 

 division of the nucleus of the cell, with especial reference to the recent 

 theories of M. Degagny. That botanist teaches that the nuclei 

 disappear progressively as the equatorial zone becomes colourable. 

 This is denied by M. Guignard, who points out that the coloration 

 of the equatorial zone is due not to nuclei but rather to the cyto- 



* MT. Zool. Stat. Neapel, vi. (1885) pp. 286-96. 



t This section is limited to papers relating to Cells and Fibres. 



% Comptcs Eendus, eii. (18S6) pp. 1036-8. 



