422 A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



of a thick sheet of protoplasm interspersed with a single layer of nuclei. This 

 endodermic layer is inclosed by an irregular membranous layer of mesoderm. 



A series of sections through the larger structure, which is interpreted as the 

 amniotic vesicle, is shown in figures 29 to 35. This vesicle, the inside diameters of 

 which are 0.1 by 0.1 by 0.06 mm., consists of an ectodermic layer clearly sub- 

 divided into embryonic ectoderm and amniotic ectoderm. The embryonic ectoderm 

 forms a sharply marked-off plate on the side towards the body-stalk of the larger 

 embryo. Like the yolk-sac, it is surrounded by a membranous sheet of mesoderm 

 in which here and there are vesicular arrangements of its cells, in some instances 

 closely resembling empty young blood-vessels. 



In accepting this structure as a monozygotic twin, the discrepancy in size 

 between it and the principal embryo suggests the possibility of its being stunted. 

 Whether it is essentially normal in form and simply retarded in its development 

 can only be determined by comparison with a much larger group of specimens than 

 is to be found in the literature up to the present time. The amniotic vesicle seems 

 to be well preserved and shows relatively normal differentiation. It corresponds in 

 many respects to those seen in the ova described by Peters, Fetzer, Jung, and Strahl 

 and Beneke. Our specimen differs from these, however, in the complete detachment 

 of the yolk-sac. This condition might justify us in considering it as abnormal. 

 It will be recalled, however, that the amniotic vesicle and the yolk-sac were as 

 widely separated in the Bryce-Teacher ovum which has hitherto been regarded as 

 normal. Inasmuch as the twin in the Mateer specimen is at least stunted and 

 possibly abnormal, it would cast some doubt upon the probability of a detached 

 yolk-sac ever being a normal condition. 



If the pregnancy had not been terminated in this case it is probable that the 

 larger embryo would have gone on to maturity, and that the smaller one would have 

 remained stationary in the form of two small epithelial vesicles, and so would have 

 been entirely overlooked. Careful search at the placental attachment of the 

 umbilical cord might frequently reveal the presence of similar minute epithelial 

 vesicles, the remains of stunted twins, and thus we might find that the tendency 

 toward twinning in man is even greater than is now supposed. 



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