THE GERM-CELLS 45 



in the act of fertilization the combination of both elements, 

 male and female, were an essential, both being, as it were, 

 complementary to each other. But this is by no means 

 the case. Male and female nuclei are not two different, 

 halves producing together the new offspring, but both are i 

 equal, each representing by itself a complete individual. ' 

 This will at once become apparent from the fact that for 

 the purpose of embryogenesis (development of the embryo) 

 either of the germ-nuclei may in certain circumstances be 

 dispensed with. 



Thus Professor Delage has shown that non-nucleated 

 fragments of eggs of the sea-urchin (Echinus) can be 

 successfully fertilized by the sperm and develop into an 

 embryo, showing thereby that the presence of the ovum 

 nucleus is not a condition sine qua non of embryonic 

 development. On the other hand, that the sperm-nucleus 

 can be dispensed with appears from some experiments of 

 Professor Loeb. He induced ova of the sea-urchin Arbacia 

 to develop parthenogenetically i.e., without the intro- 

 duction of a spermatozoon by bringing them for a 

 short time into a mixture of 50 per cent, sea- water and 

 50 per cent, magnesium chloride. 



The cases of partial and total Parthenogenesis already 

 mentioned also point to the same conclusion that the egg- 

 cell alone is capable of starting its own embryogenesis. 



We have thus in fertilization a twofold process : firstly, 

 a stimulus given to the germ-cell towards embryogenesis, 

 this stimulus being supplied either by the germ- cell of the 

 opposite sex or by some other means ; and, secondly, 

 ajnphimixis i.e., the co-mingling of the hereditary sub- 

 stances of the two parent-nuclei. 



V.DE VELOPMENT. 



In accordance with their theories of fertilization, the 

 Ovists and Animalculists held that the future embryo was 

 contained in the ovum or spermatozoon respectively. They 



