804 EMBRYOLOGY 



Soon after the development of the amnion the allantois is formed. 

 This membrane is vascular. It encroaches upon and takes the place of 

 the external amniotic membrane, and is covered with hollow villi, which 

 take the place of the villi of the amnion. Over a certain portion of the 

 membrane the villi are permanent. The mode of development of the 

 amnion is illustrated by the diagrammatic Fig. 224. This figure 

 illustrates, the formation of the amnion, the umbilical vesicle and the 

 allantois. The last two structures are derived from the hypoblast and 

 the internal layer of the mesoblast. 



When the allantois has become the chorion, or the external membrane 

 of the ovum, having taken the place of the external layer of the amnion, 

 the structures of the ovum are the following: I. The chorion, formed 

 of the two layers of the allantois and penetrated by bloodvessels. 

 2. The umbilical cord, which connects the embryo with the placental 

 portion of the chorion, and the umbilical vesicle, formed from the same 

 layers as the allantois. 3. The amnion, which is the internal layer of 

 the amniotic fold, persisting throughout foetal life. 4. The embryo 

 itself. 



During the early stages of development of the umbilical vesicle and 

 the allantois, the internal amniotic layer, or the true amniotic membrane, 

 is closely applied to the surface of the embryo and is continuous with 

 the epidermis at the umbilicus. It is then separated from the allantois 

 by a layer of gelatinous matter ; and in this layer, between the amnion 

 and the allantois, is the umbilical vesicle. At this time the umbilical 

 cord is short and not twisted. As development advances, however, the 

 intermembranous gelatinous matter gradually disappears ; the cavity of 

 the amnion is enlarged by the production of liquid between its internal 

 surface and the embryo ; and at about the end of the fourth month the 

 amnion cornes in contact with the internal surface of the chorion. At 

 this time the embryo floats in the liquid contained in the amniotic 

 cavity. 



The amnion forms a lining membrane for the chorion. By its 

 gradual enlargement it has formed a covering for the umbilical cord ; 

 and between it and the cord, is the atrophied umbilical vesicle. The 

 amnion then resembles a serous membrane, except that it is non-vascular. 

 It is lined with a single layer of pale delicate cells of pavement-epithe- 

 lium, which contain a few fine fatty granules. At term the amnion 

 adheres to the chorion, although it may be separated with a little care 

 as a distinct membrane and may be stripped from the cord. From its 

 arrangement and from the absence of bloodvessels, it is evident that this 

 membrane is simply for the protection of the foetus and is not directly 

 concerned in its nutrition and development (see Plate X, Fig. 6). 



