146 Edward Hindle 



cleavage centrosomes were supposed to arise from the achromatic 

 part of the nucleus and the chromosomes from the nucleolus. 



MORGAN ('96, '99, 1900) found that the cytoplasm of eggs of 

 Arbacia, when treated with hypertonic solutions of NaCl and MgCl 2 , 

 or strychnine, became filled with artificial astrospheres each con- 

 taining a centrosome. The centrosomes were therefore considered 

 to arise de novo and MORGAN expressed agreement with HERT- 

 WIG'S view that they may develop out of the achromatic part of the 

 nucleus. 



In 1901 appeared WILSON'S important paper on the cytological 

 changes following the chemical fertilization of the- eggs of Toxo- 

 pmustes variegatus by means of LOEB'S older method, in which 

 magnesium chloride was the agent employed. He found that, in 

 accordance with LOEB'S observations, the eggs vary considerably in 

 their response to the hypertonic solution, even when derived from the 

 same female and lying side by side in the same solution. He goes 

 on to say (p. 531): First, even those eggs that segment and give 

 rise to swimming larvae show wide differences in both the rate and 

 character of the internal changes and in the ensuing form of cleavage. 

 Second, a great variety of asymmetrical mitoses and other patholog- 

 ical phenomena, leading to the production of monstrous forms always 

 occur in a considerable number of the eggs. Third, numerous gra- 

 dations of the mitotic processes occur, the incomplete mitoses show- 

 ing innumerable modifications ranging from a hardly perceptible 

 change up to a nearly complete division. As a result of his exam- 

 ination, WILSON showed that both cleavage-asters and cytasters to- 

 gether with their centrosomes may arise de novo. Further, the 

 cleavage aster arises from the nucleus and its centrosome is more 

 distinct than that of the cytasters. The chromosomes were found to 

 arise in two different ways; in one case, they arise in the ordinary 

 way direct from the reticulum, whereas in the other case they are 

 built up by the nucleolus. Moreover only eighteen chromosomes were 

 found instead of the normal number of thirty six. 



In the same year DELAGE (1901) published the results of his 

 cytological study of artificial parthenogenesis in the eggs of Strongylo- 

 centrotus lividus. His results differed markedly from those of WILSON, 

 for the full number of chromosomes was stated to be restored in the 

 artificial embryos, DELAGE having found nine chromosomes in the 

 unfertilized ovum of this species and eighteen in the cells of the 

 parthenogenetic larvae. 



