146 ARTIFICIAL PARTHENOGENESIS [CH. 



of the egg, rather than to the introduction of any substance 

 into its interior, but there seems to be no reason against 

 accepting this conclusion and at the same time regarding 

 the initiation of development as depending on factors which, 

 if unrestrained, lead on to the cytolysis of the egg as a whole. 



It should also be noted that DELAGE (1908) has brought 

 about artificial parthenogenesis by a method based on 

 different reasoning. He considers that the rhythm of cell- 

 division is based on a rhythmic cycle of alternate coagula- 

 tion and liquefaction of the protoplasmic colloids, the 

 formation of the centrosome, spindle and asters being due 

 to coagulation, while the dissolution of the nucleus and 

 splitting of the chromosomes are examples of liquefaction. 

 He therefore subjected eggs to the successive influence of 

 an acid and an alkali, since acids coagulate and alkalis 

 liquefy colloids. By these means he produced very success- 

 ful parthenogenetic development in Sea-urchin eggs, a 

 result which seems to confirm his theoretical interpretation. 



The changes undergone by the nucleus and cytoplasm of 

 artificially parthenogenetic Echinoid eggs are closely similar 

 to those seen in eggs fertilised by a spermatozoon. Slight 

 preliminary changes occur in the interval between the 

 treatment with butyric acid and the hypertonic solution; 

 these are suspended while the eggs are immersed in the 

 hypertonic solution, but as soon as they are replaced in 

 sea-water the nucleus begins to enter the prophase stage of 

 mitosis, and shortly afterwards an aster appears at one pole 

 of the nucleus, the rays of which radiate from a diffuse 

 centrosome close to the nuclear membrane. The centrosome 

 then divides, its halves diverge and a normal spindle is 

 formed between them. The chromosomes have by this 

 time made their appearance in the nucleus, the membrane 

 disappears, and the chromosomes take up their position in 



