DEVELOPMENT OF CENTRIFUGED EGGS AND FRACTIONS I4I 



develop quite like the fertilized red half. They develop best if activated 

 just after centrifuging while still elongate, but spherical ones can also 

 be activated. The fertilization membrane and hyaline layer are thick, 

 as in the fertilized red halves. A clear sphere resembling a nucleus is 

 often present 20 minutes after activation; it is difficult to say whether 

 there is a membrane around it, but there seems to be a phase bound- 

 ary. The monaster which is often very conspicuous, arises near the 

 "pseudonucleus." The monaster stage is followed by an amphiaster, 

 and a cleavage plane may come in between the two asters in typical 

 fashion. It seems curious that there should be a monaster stage pre- 

 ceeding the amphiaster when there is no nucleus. The first 

 cleavage plane may divide the egg equally, in any relation to the 

 stratification though more often parallel with it. In elongate eggs the 

 first cleavage plane comes across the short axis as in other types of 

 elongate eggs and half-eggs. Successive cleavages may be fairly regular 

 and multicellular organisms of some 500 cells have been obtained. Often 

 cell walls fail to come in, and in later stages the egg is peppered with 

 asters and small spheres which resemble nuclei. Development is very 

 slow and they hatch only after 24 hours, whereas the whole egg hat- 

 ches in 8 to 9 hours; the fertilized red half hatches in 11 hours. No 

 further development has been obtained ; there is apparently no growth 

 or differentiation without nuclear material, though these organisms 

 have lived for a month ; unfertilized eggs live at room temperature only 

 a day or two. Many substances have been added to the sea water in 

 an effort to get further development of the parthenogenetic merogones 

 without success: killed Arbacia sperm, living frog nuclei, nuclei acid, 

 phage, auxin, adenine, guanine, uracil. 



In stained sections there are of course no chromosomes, and there are 

 no spindles, but there are large asters arranged in pairs with very coarse 

 rays (Plate XVI, Photograph 13). The Feulgen reaction, specific for 

 chromatin, was negative for the parthenogenetic merogones; there was 

 no red staining material, whereas the fertilized merogones prepared 

 in the same way at the same time showed it very clearly. A comparison 

 of the blastula of a fertilized merogone and a parthenogenetic mero- 

 gone in aceto-carmine preparations shows very clearly the presence of 

 chromosomes in the former, and their absence in the latter (see E. B. 

 Harvey, 1936, 1938a, 1940b, c, 1951, Figs. 72, 73). 



Compare photographs on Plate XI with corresponding ones on 

 Plate X to see how similar is the development of parthenogenetic mero- 

 gones and fertilized merogones up to a certain stage. 



