APPROACH AND CONJUGATION OF THE NUCLEI, ETC. 



19 



sections in various planes shows tliat the central mass really has the form of a cap, extending downwards in all directions 

 from the sperm-nucleus, which lies at its apex, and is often actually embedded in its substance. The sperm-nucleus is 

 considerably enlarged and its substance is becoming loose and reticulated (Text-figs. XI. B ; XII. A). 



Plate V. Phototype 19. 



The Sperm-aster has divided. A^neiei fusing (rj minutes). 



The sperm-nucleus has flattened down against the egg-nucleus (the contour of which is not quite in focus); and 

 appears as a dark lens-shaped body, having a closely reticular structure (cf. Text-fig. XII. B). The sperm-aster has 

 completely divided into two haKes which lie at opposite poles of the egg-nucleus. The figure clearly shows the fact that 

 the astral rays from the two sides cross one another nearly at a right angle opposite the equatorial region of the nucleus. 



Fig. XII. 

 Fig. XII. — Conjugation of the nuclei. Extension and division of the sperm-aster (looo diameters). 



A. The two nuclei here lie in contact, the sperm-nucleus considerably enlarged and showing a reticular structure. The sperm-aster has enormously increased; its central mass extends 



like a cap far down upon the egg-nucleus, and its rays traverse nearly an entire hemisphere of the egg. (Phototype iS. ) 



B. The sperm-aster has divided into two. The sperm-nucleus is flattened against the egg-nucleus forming a lens-shaped bodv. (Phototype 19.) 



This fact indicates that the rays are really fibres, and not, as some recent authors have maintained, merely the optical 

 sections of thin plates or lamella' in a radially arranged alveolar structure. 



In the European sea-urchin. Echinus niicrotubcrculatus, Boveri describes both the germ-nuclei as containing, at this 

 period, nine elongated chromosomes. In Toxopneustcs, the chromosomes cannot be distinguished until a much later 

 period, and both nuclei consist of an irregular reticulum of chromatin. 



Pl.\te V. Phototype 20. 



Nuclei fusing. Sperni-mccleus en face. 



A stage just later than the last viewed from the side of the sperm-nucleus. The sperm-nucleus appears as a dark 

 irregular mass at the left upper side, its boundary having been nearly lost. The chromatin-granules of the egg-nucleus 

 are distinctly shown. 



