Ill PARTHENOGENESIS 95 



with a spermatozoon. The accurate study of this phenomenon dates 

 from the experiments of O. and R. Hertwig on Echinoderm eggs (1887). 

 The methods have been specially elaborated by Loeb and others, and 

 consist, so far as the eggs of marine animals are concerned, in placing 

 them in sea water of which the chemical composition has been altered 

 in various ways ; for details the reader is referred to any of the works 

 cited below. 



From the point of view of cytology, two points claim special attention 

 — the achromatic figure and the nucleus. As we have already seen, in 

 the mature egg the centrosome and other parts of the achromatic figure 

 apparently disappear ; a new centrosome, which in turn gives rise to 

 the rest of the figure, being provided for the zygote by the spermatozoon. 

 Eggs which have started development under the stimulus of chemical 

 reagents are, however, provided with typical centrosomes and spindle 

 figure, etc. Two alternatives as to the origin of these are possible : (i) 

 that the old egg centrosome does not disappear entirely, but is merely 

 rendered latent and is reawakened to activity by the stimulus which 

 induces the parthenogenesis ; or (2) that the centrosomes arise de novo 

 in the stimulated egg. Although the first alternative may be true in 

 some cases, it has been demonstrated beyond question that the latter 

 may occur also. Wilson (1901) showed this for Toxopneustes. If the 

 eggs of this sea-urchin are subjected to sea water to which MgClg has 

 been added, a large number of division centres [cytasters) may appear 

 simultaneously in the cytoplasm. Each centre is provided with a 

 centrosome and rays, exactly as in a typical aster. These asters divide 

 again like normal asters, before nuclear division, their division being 

 preceded by that of the centrosome. Both Wilson and M'Clendon (1909) 

 even obtained cytasters in eggs from which the nucleus had been 

 removed. Such enucleated eggs of Aster ias segmented for several 

 hours, dividing into a large number of irregular blastomeres by the 

 action of these cytasters. We are therefore compelled to conclude that 

 the centrosome and its derivatives, though normally a permanent cell 

 structure derived by division of a previous one, can arise de novo in the 

 cytoplasm. 



As regards the nuclear cytology of artificial parthenogenesis, we 



must distinguish between the types of eggs mentioned on p. y^x accord- 



. ing as to whether maturation normally takes place before or after 



entry of the spermatozoon or, in the case of artificial parthenogenesis, 



the stimulus which takes the place of this. 



The simplest case is afforded by those eggs which mature before the 

 entry of the spermatozoon ; for example, those of the Echinoidea. These 

 eggs mature in the ovary, and therefore when, after being laid, they are 

 subjected to the stimulus which is to cause them to develop, they are 



