EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 83 



functions performed by the yolk-sac in birds, with a consequent degeneration of 

 the ungulate yolk-sac. In man and Unguiculates, the functions of the allantois 

 are transferred to the chorion and the allantois in turn becomes a rudimentary 

 structure. 



The Amnion. This is formed precociously in Unguiculates and in a manner 

 quite different from its mode of origin in Ungulates and birds. It is assumed that 

 its cavity arises as a split in the primitive ectoderm of human embryos, as in bat 

 embryos (Fig. 71). 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. 70 D). It becomes 



m.pl. 



^^r 



FIG. 72. Section of embryonic rudiment in Peters' ovum (first week) (after Peters), ect, ectoderm 

 of chorion; mes, mesoderm; am, amnion; em. pi., embryonic plate; y.s., yolk-sac; ent, entoderm; 

 ex. CCK., portion of extra-embryonic ccelom limited by a strand of the magma reticulare. 



a thin, pellucid, non-vascular membrane and about a month before birth is in 

 contact with the chorion. It then contains from one-half to three-fourths of 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. The embryo is protected from maceration by a white fatty secretion, the 

 vernix caseosa. 



At birth the amnion is ruptured either normally or artificially. If not ruptured, 

 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 may extend across the amniotic cavity and, pressing upon parts of the embryo 



