No. 1.] ORIGIN OF HUMAN MONSTERS. 35 
article of Von Recklinghausen” upon spina bifida as a repre- 
sentative one for man. 
About the same time Morgan and Tsuda,* in working upon 
the orientation of the frogs’ eggs, subjected them to a great 
variety of solutions, and found that a .6 per cent solution of 
sodium chloride prevented closure of the blastopore. By them 
the nail was hit upon the head; the other investigators only 
obtained monsters occasionally (Hertwig incorrectly believed 
them to be due to polyspermy), but Morgan and Tsuda ob- 
tained them in great number. It was found that less than .6 
per cent of salt did not affect the embryo and a stronger 
solution killed it. Successful specimens, and there were many 
of them, were examined from stage to stage in their develop- 
ment and the exact steps by which the blastopore is closed 
was followed. This gave them a decided advantage in study 
over the hap-hazard one in finding embryos already formed. 
The experiments were used mainly to study the orientation 
of the embryo in its relation to the lips of the blastopore. 
The crucial experiment of Morgan and Tsuda was imme- 
diately seized upon by Hertwig* and employed in his experi- 
ments on spina bifida. Spina bifida could now be studied 
experimentally. Hertwig also found that a .6 per cent solu- 
tion of common salt delayed the development of frogs’ eggs, 
the intestines, chorda, myotomes and nervous system develop- 
ing normally, but gastrulation was postponed for from twelve 
to twenty-four hours. As a result of this the spinal cord 
does not close posteriorly as rapidly as it should and perma- 
nent spina bifida follows. Often the walls of the spinal tube 
are thin and its lumen is small, showing that there is a 
general arrest of its development. 
In general, Hertwig did not continue the experiments 
beyond the sixth day, for the salt caused marked changes in 
the exposed spinal cord. It seemed to be less resistant, inas- 
*Von Recklinghausen, Virch, Arch., 105, 1886. 
®Morgan and Tsuda, Quart. Jour. Micr. Sci., N. S. 35, 1894. Also, 
Morgan, Roux’s Archiv, pp. 266, 269 and 293, 1902, 
*Hertwig, Arch. f. mik. Anat., XLIV, 1905. 
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