No. 1.] ORIGIN OF HUMAN MONSTERS. III 
During the fourth week of development the preskeletal 
tissues make their appearance, and through this change the 
embryos naturally fall into the two groups under which I am 
discussing them. The mesenchyme also becomes more resistant 
and does not dissociate so easily. The peripheral nerves are 
well laid down and the premuscle tissue is making its ap- 
pearance. Thus we have new conditions by which we can 
recognize the time at which development was arrested; these 
naturally influence secondary changes in the embryo. In these 
specimens hydramnios is nearly always found, and since the 
embryos usually become markedly smaller when their de- 
velopment is arrested it is rational to conclude that hydram- 
nios is produced by a continued growth of the amnion and 
chorion when the development of the embryo has been ar- 
rested. I have found no evidence in favor of the theory that 
hydramnios is due to arrested development of the amnion and 
dwarfism of the embryo. Were this the case we should find 
bones and cartilages in my specimens of the fourth week. In 
the specimens given in Table IX we have four weeks’ embryos 
in eight weeks’ chorions, not degenerated eight weeks’ em- 
bryos in eight weeks’ chorions. The four good specimens 
(Nos. 347, 285, 336 and 334) of the end of the fourth week 
are of different degrees of degeneration and form an excel- 
lent continuous series. No. 347 is from an ovum in which 
the mesoderm of the chorion is fibrous, the syncytium being 
scanty and mixed more or less with pus. The changes within 
the embryo are well marked, the brain being dissociated and 
solid and the medulla and cord are dilated. The blood-vessels 
are well marked, dilated and filled with blood, which is just 
beginning to infiltrate into the surrounding tissues. The other 
organs are just beginning to dissociate. The precartilage is 
well marked, however, and the rest of the mesodermal tissue 
has in it many round cells which seem to have come from the 
blood. Many of these cells are in the pleuro-peritoneal cavity. 
Conditions are advanced markedly in No. 285. Here the 
menstrual history tells us that the pathological process has 
been under way for a number of weeks. The mesoderm of 
