76 HUMAN EMBRYOS AND FETAL MEMBRANES 



It is assumed that its cavity arises as a split in the primitive ectoderm of 

 human embryos, as in bat embryos (Fig. 75). Later, a somatic layer of 

 mesoderm envelops its ectodermal layer, its component parts then being 

 the same as in birds and Ungulates an inner layer of ectoderm and an 

 outer layer of mesoderm (Fig. 74 D). It becomes a thin, pellucid, non- 

 vascular membrane, and about a month before birth is in contact with the 

 chorion. It then contains about a liter of amniotic fluid, the origin of 

 which is unknown. During the early months of 'pregnancy the embryo, 

 suspended by the umbilical cord, floats in the amniotic fluid which serves 

 as a water cushion. The embryo is protected from maceration by a white, 

 fatty secretion, the vernix caseosa. 



At birth the membranes rupture. If the chorion bursts alone, the child may be born 

 enveloped in the amnion, popularly known as a veil, or 'caul.' The amniotic fluid may be 

 present in excessive amount, the condition being known as hydramnios. If less than the 

 normal amount of fluid is present, the amnion may adhere to the embryo and produce 

 malformations. It has been found, too, that fibrous bands or cords of tissue sometimes 

 extend across the amniotic cavity, and, pressing upon parts of the embryo during its growth, 

 cause scars and splitting of eyelids or lips. Such amniotic threads may even amputate a 

 limb or cause the bifurcation of a digit. 



Amniotic cavity 

 Mesoderm of chorionic villus 



Ectoderm of chorion 



rm of embryo 

 Mesoderm 



Extra-embryonic coelom 



Yolk sac 

 Mesoderm 



Entoderm 



FIG. 76. Section of Peters' embryo of 0.2 mm. (about fifteen days). The portion of extra- 

 embryonic ccelom shown is limited below by a strand of the magma reticulare. 



The Allantois. The allantois appears very early in the human em- 

 bryo, before the development of the fore-gut -or hind-gut. In Peters' 

 embryo the amnion, chorion, and yolk sac are present, but not the allan- 

 tois (Fig. 76). In an embryo 1.54 mm. long, described by von Spec 

 (Fig. 77), there is no hind-gut, but the allantoic diverticulum of the en- 



