GENERATION AND INHERITANCE 



291 



sexes are separate, and as the eggs which almost the whole year 

 through may be found in the mature condition are small and trans- 

 parent, it is here an easy task to observe the artificial fertilization 

 on the object-glass and under the microscope. The complete trans- 

 parency of the egg permits, even with extreme magnification, the 

 most minute processes to be observed during life. That which has 

 already been discovered can be controlled and more accurately deter- 

 mined in many details in preserved material. 



Thus the important points of the process of fertilization could be 

 explained and later positively determined by me and the numerous 

 investigators who have since then occupied themselves with the 

 Echinodermata (Diagram I). 



DIAGRAM I. The fertilization process in the ovum of Toxopneustes lividus. 



FIG. 1 . The mature egg at the moment of fertilization. Of the numerous sperma- 

 tozoa one has already penetrated the egg at a point which is determined by the 

 "reception eminence." In the spermatozoon the head (&), the middle piece (m), 

 and the terminal filament may be distinguished. The egg-nucleus (eik). 



FIG. 2. The egg a few minutes later has excreted the yolk-sac (Membrana vitel- 

 lind). The head and the middle piece have separated from the terminal filament, 

 which has disappeared, and have changed into the male pro-nucleus (sfc) and the 

 centrosome (c). The latter is surrounded by protoplasmic rays. The distance 

 between the sperm-nucleus and the egg-nucleus has lessened. 



FIG. 3. A few minutes later. The egg- and sperm-nuclei have approached one 

 another in the centre of the ovum. The originally simple centrosome has divided 

 in two. The protoplasmic rays around the two nuclei have become larger. 



FIG. 4. The egg- and sperm-nuclei lie against one another and have become flat- 

 tened at the place of contact. The centrosomes are arranged on opposite sides 

 of the nuclei. The protoplasmic rays have spread themselves out over the entire 

 yolk. 



