80 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



cord (Fig. 172). At the 42 mm. stage, according to Lewis and Mall, the gut re- 

 turns to the ccelom of the body. The mucous tissue peculiar to the cord arises 

 from mesenchyme. It contains no capillaries and no nerves, but embedded in it 

 are the large umbilical vein, the two arteries, the allantois and the yolk-stalk. 

 The umbilical cord may become wound about the neck of the fetus, causing its 

 death and abortion, or by coiling 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. 16 and 17), we 

 find it consists of an outer layer, which we have called the trophectoderm, and the 

 inner cell mass. 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 earliest 

 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 guess at their early origin by what we know from other mammals. The 

 diagrams (Fig. 70 A and B) show two hypothetical 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 trophoderm. It is assumed that as the 

 embryo grows (Fig. 70 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 (Fig. 70 C), the extra-embryonic mesoderm has 

 extended between the trophectoderm and the ectoderm of the amnion and the 

 extra-embryonic ccelom appears. The amniotic cavity has increased in size and 

 the embryo is attached to the trophectoderm by the unsplit 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 villi surrounded 

 by trophoderm cells. In the fourth stage, based on Graf Spec's embryo (D), 

 the chorionic villi are longer and branched. The mesoderm now remains unsplit 

 only at the posterior end of the embryo, where it forms the body-stalk peculiar 



