THE AMNION. 91 



Germinal area. In the mass of nucleated cells into which the yelk becomes 

 converted during the formation of the blastodermic vesicle, a small agglome- 

 ration is formed, which then spreads out into 

 an area of nucleated cells, from which the em- Fig. 52. 



bryo is to be formed, and which has accord- 

 ingly received the name of germinal disk or 

 area germinativa. In this portion of the ovum 

 the first trace of the embryo appears as a faint 

 streak, which is called the primitive trace or 

 primitive groove. This groove first deepens into 

 a furrow, bounded by two plates the laminse 

 dorsales, beneath which a delicate fibril appeals 

 the chorda dorsalis or notochord in which 

 cartilage can very early be recognized, and 

 which forms the future spinal column. 



The germinal disk is found to consist on a 

 transverse section of three layers; an upper STY? iT 



,. , - ,. ' profile to show the - 



(external), or serous; a lower (internal), or todermic memhrane . i. viteiiine mem- 



mUCOUS ; and a middle layer, which IS formed brane. 2. Blastoderm. Germinal area, 

 from the mUCOUS lamina, as above shown. 4. Place where the blastoderm is just di- 



The chorda dorsalis and the laminae dorsales vided into its two layers, 

 are the rudiments of the vertebral column and 



canal. The upper or serous layer of the embryo gives origin to the cere- 

 bro-spinal nervous centres, and to the organs of the senses, including the 

 cuticle and its appendages, as also to the mammary glands. From the middle 

 layer are developed the locomotive organs, the spinal and sympathetic nerves, 

 the vascular system, the ductless glands, the sexual organs, the cutis, where 

 the middle layer touches on the external, and the muscular and submucous 

 coats of the intestines, where it touches on the internal layer. The latter fur- 

 nishes the lining of the alimentary canal and its various appendages, liver, 

 pancreas, &c., the respiratory organs, and the urinary organs. Besides this, 

 however, there are three appendages to the ovum, which must now be described 

 as the amnion, the umbilical vesicle, and the allantois. 



Formation of the Amnion. The amnion is formed from the external germinal 

 layer, which is drawn in on all sides by the changes of shape of the embryo. 

 The embryo as it grows becomes curved at its anterior and posterior end, so 

 as to form the cephalic and caudal flexures (Fig. 54, B); it also curves on itself 

 laterally towards the umbilicus (Fig. 53, 7), and as it does so, it draws the ex- 

 ternal germinal layer with it, forming double folds which meet at the umbili- 

 cus, and at a point opposite to the umbilicus on the dorsal aspect of the embryo, 

 sometimes called "the posterior umbilicus" (Fig. 54, 8'), and finally communi- 

 cate so as to form a delicate closed sac, into which a serous fluid the liquor 

 amnii is secreted. This fluid increases in quantity up to about the end of 

 the fifth or the sixth month, when it reaches the amount of about two pints. 

 Thence it diminishes, and at the end of pregnancy is about half its maximum 

 quantity. The outer layer of the amnion incloses all the parts of which the 

 embryo consists, and is in contact externally with the chorion. 



The portion of the external germinal layer which does not take part in the 

 formation of the arnnion is called the vesicula serosa (Fig. 54, 2'). When the 

 sac of the amnion is completely closed, the vesicula serosa becomes detached 

 from it, and then forms an envelope to the ovum, lining the primitive chorion. 

 Its future destination appears to be to form the epithelial layer of the secondary 

 or permanent chorion. 



The allantois (Fig. 54, al) is a projection from near the hinder part of the 

 embryo, formed by the middle and internal germinal layers, and therefore con- 

 tinuous with the intestinal cavity. The lower part of this cavity becomes the 



