204 LIFE: ITS NATURE AND ORIGIN 



become fastened by a sticky secretion of certain larval glands, to the 

 probosis of some female worm, where their gradual development into 

 males can be followed. Those who have experimented with this 

 rather rare worm, speak of the number of "probosis hours" in the life 

 of such a male. 



Artificial Parthenogenesis 



Recent research in artificial parthenogenesis has developed 

 facts of great interest here. E. B. Harvey 48 obtained by ultra- 

 centrifugation nuclei-free fragments of sea-urchin eggs which, 

 when activated by hypertonic seawater 49 in many cases gave rise 

 to a single cy taster (and frequently to several cy tasters). The 

 cytaster was followed by an amphiaster and division of the cell 

 between the two asters. Afterward, when cell division does not 

 carry through, the cytasters multiply within the unsegmented 

 fragment. Sometimes the merogens develop as far as the 500-cell 

 stage and form free-swimming blastulae, before development 

 ceases. The egg cytoplasm evidently carries sufficient directive 

 agents for the early stages of development, but chromatin or 

 nuclear material appears necessary for gastrulation and further 

 differentiation. J. Needham comments that the failure of these 

 parthogenetic merogens to show the Feulgen reaction (used to 

 identify nuclear material) is inconclusive, for though there must 

 be much of this ribo-nucleotide type nuclear material in the 

 cytoplasm anyway, it cannot bring about gastrulation; neither 

 has the external addition of nucleotides, vitamins or hormones. 



Waiving the question as to the possible presence of dispersed 

 nuclear material in the non-nucleated merogen, A. Tyler 50 be- 

 lieves that in normal mitoses asters arise in connection with, and 

 are probably due to, the activity of a self-perpetuating central 

 body, even though Harvey 51 did not describe definite central 

 bodies in the cytasters. For as far back as 1901 E. B. Wilson, 52 

 figured them quite clearly in his cytological observations on ac- 

 tivated non-nucleated fragments; and more recently Fry 53 showed 

 that differences in cytological technique may greatly affect the 

 appearance of the central body. Therefore Tyler believes "it 

 is reasonable to assume that the appropriate method would dem- 

 onstrate distinct central bodies in Harvey's material too. It ap- 

 pears then that the production of a cytaster involves in effect the 

 de novo formation of a self-multiplicative, genetically continuous 

 body." 



