74 



THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



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 Hter 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 



mes. am. 



em. pi. 





^•' 



Fig. 76. — Section of Peter's embryo of 0.2 mm. (about fifteen days), ed., Ectoderm of chorion; 

 mes., mesoderm; am., amniotic cavity; em. pi., embryonic plate; y.s., yolk sac; cnt., entoderm; c.\. coe., 

 portion of extra-embryonic coelom limited by a strand of the magma reticulare. 



amniotic fluid may be present in excessive amount, the condition being known as hydram- 

 nios. 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 

 during its growth, may cause scars and splitting of eyelids or lips. Such amniotic threads 

 may even amputate a limb or cause the bifurcation of a digit. 



The AUantoisv — The allantois appears very early in the human embryo be- 

 fore the development of the fore-gut or hind-gut. In Peter's embryo the amnion, 

 chorion, and yolk sac are present, but not the allantois (Fig. 76). In an embryo 

 1.54 mm. long, described by Von Spee (Fig. 77), there is no hind-gut, but the 



