EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 71 



arises from mesenchyme. It contains no capillaries and no nerves, but embed- 

 ded in it are the large umbihcal vein, the two arteries, the allantois, and the yolk 

 stalk. The umbilical cord may become wound about the neck of the fetus, caus- 

 ing its death and abortion, or by coihng about the extremities it may lead to their 

 atrophy or amputation. 



EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 



Referring to the blastodermic vesicle of the mammal (Figs. 17 and 18), it 

 is found to consist of an outer layer, which we have called the trophectoderm, and 

 the inner cell mass (p. 36). The trophectoderm forms the primitive ectodermal 

 layer of the chorion in the higher mammals and probably in man. From the inner 

 cell mass are derived the primary ectoderm, entoderm, and mesoderm. In the 

 earhest known human embryos described by Teacher, Bryce, and Peters, the germ 

 layers and amnion are present, indicating that they are formed very early. We 

 can only infer their early origin from what is known of other mammals. The 

 diagrams (Fig. 74 A and B) show two h}rpothetical stages seen in median longi- 

 tudinal section. In the first stage {A) the blastodermic vesicle is surrounded by 

 the trophectoderm layer. The inner cell mass is differentiated into a dorsal mass 

 of ectoderm and a ventral mass of entoderm. Mesoderm more or less completely 

 fills the space between entoderm and trophectoderm. It is assumed that as the 

 embryo grows {B) a split occurs in the mass of ectoderm cells, giving rise to the 

 amniotic cavity and dividing these cells into the ectodermal layer of the embryo 

 and into the extra-embryonic ectoderm of the amnion. At the same time a 

 cavity may be assumed to form in the entoderm, giving rise to the primitive gut. 

 About this stage the embryo embeds itself in the uterine mucosa. In the third 

 stage, based on Peter's embryo (C), the extra-embryonic mesoderm has extended 

 between the trophectoderm and the ectoderm of the amnion and the extra- 

 embryonic coelom appears. At first strands of mesoderm, known as the magma 

 reticulare, bridge across the coelom between the somatic and splanchnic layers of 

 mesoderm (Fig. 76). The amniotic cavity has increased in size and the embryo 

 is attached to the trophectoderm by the unspht layer of mesoderm between the 

 ectoderm of the amnion and the trophectoderm of the chorion. The latter shows 

 thickenings which are the anlages of the chorionic vilU surrounded by trophoderm 

 cells. In the fourth stage, based on Graf Spee's embryo (D), the chorionic viUi 

 are longer and branched. The mesoderm now remains unspht only at the pos- 

 terior end of the embryo, where it forms the body stalk pecuUar to Unguiculates 

 and Primates. It connects the mesoderm of the embryo with the mesoderm of the 



