964 7. MERCURIALS 



containing all the pigment, this fragment later cytolyzing. Hoadley claimed 

 that no true cleavage occurs and called the Hg++-induced behavior pseu- 

 docleavage. Both Heilbrunn and Hoadley felt that the Hg++ acts primarily 

 on the cortical region, and Hoadley, in addition, thought that the Hg++ 

 may react with the pigment itself. 



Kriszat and Runnstrom (1952) reported a strange phenomenon occurring 

 in Arhacia eggs treated with 0.028 mill p-MB. This concentration of mer- 

 curial rapidly inactivates the spermatozoa, but some fertilization can occur 

 before this is complete. Fertilization causes a strong contraction of the cor- 

 tical layer, squeezing out the cytoplasm into a number of pigment-free 

 lobes, the cortex, containing all the pigment, shrinking to a small folded 

 sac. It was postulated that p-MB blocks rather specifically those processes 

 reversing the surface contraction occurring during normal fertilization, it 

 is quite possible that these effects are exerted directly on the SH groups 

 of the protein components of the cortex (or plasma membrane, since it is 

 difficult to differentiate them), rather than on enzymes. The concentration 

 of mercurial is very critical. Hg++ at 0.02 n\M acting for 20 min on Arhacia 

 eggs prevents development beyond the early blastula stage, but acting for 

 6 min has no effect on motility or larvae; however, 0.025 vaM acting for 

 3 min reduces cleavage and interferes with development (Hoadley, 1930). 

 If the mercurial is added some time after insemination, the effects are 

 modified. Thus 0.05 vaM p-MB 30 min after fertilization scarcely interferes 

 with cleavage, the delay in onset being only 1-2 min and 95% of the eggs 

 dividing (Zimmerman et al., 1957). Mersalyl is less inhibitory and at 2 mM 

 the eggs cleave normally, although there is a 10-15 min delay; only 40% 

 develop to the blastula stage. PM, on the other hand, is very potent, indi- 

 cating possible permeability factors (Macfarlane and Nadeau, 1948). De- 

 velopment of Tripneustes esculentus (sea urchin) embryos, exposed at the 

 2-4 cell stage for 1 hr to 0.001 mill PM, is inhibited and only 2% reach a 

 motile blastula stage. Many of the embryos are abnormal and partial cytol- 

 ysis occurs. Even 0.00038 m3I PM slows yolk absorption although cleavage 

 is not affected. Echinus miliaris larvae exposed to 0.0005 mill Hg++ meta- 

 morphose, but there is dedifferentiation of the tissues so that the young 

 echinus is often abnormal, perhaps possessing rudimentary tube-feet or 

 spines (Huxley, 1928). Gastrulation is a process generally sensitive to toxic 

 substances and this is true for the mercurials. For example, frog dorsal lip 

 explants are depressed rapidly by 0.1 milf p-MB so that little further de- 

 velopment takes place (Ornstein and Gregg, 1952; Gregg and Ornstein, 

 1953). The mercurial seems to prevent certain movements and spreading 

 associated with gastrulation, e.g., the stretching of the mesoderm within 

 the endoderm and the ectodermal flow over the endoderm. Not much has 

 been done on later embryonic development, but p-MB injected into newborn 

 mice brings about varying degrees of neuroblastic necrosis, chiefly in the 



