Embryology. 15 



darker int<Tn;il cells now collecting first into a mass and then into a 

 plate or round disc on one side of the hollow interior. This spot is 

 called the germ area or germ disc, and when fully formed it consists of 

 the two single layers of cells, the outer or exoderm cells which go clear 

 round the globe, and the small inner layer or entoderm, covering only a 

 fraction of the inner circumference of the globe. The place inside the 

 globe not occupied by the entoderm cells becomes filled with a bright, 

 transparent liquid. This process has all gone on within the original egg 

 covering or zona pellucida with its mucous coating, before mentioned, 

 and to which the name prochorion is given. This prochorion now in- 

 creases in size more rapidly than the enclosed egg, thus leaving a space 

 between the two which becomes filled with fluid. On the outside of the 

 prochorion there are developed numerous protuberances or sprouts which 

 mark the places where afterwards the tufted processes or villi appear 

 which arc to connect the chorion with the uterus. 



FIG. 22. Diagrams of develop- 

 ment of mammal embryo and its 

 membranes. Longitudinal verti- 

 cal section. 



No. 1. a. Outer germ layer, i. 

 Inner germ layer, ra. Middle 

 germ layer, developed from the 

 other t\vo. kit. Intestinal germ 

 vesicle or yelk. d Prochorion 

 with its tufts d'. 



No. 2. c. The embryo begin- 

 ning 1 o be separated from the germ 

 vesicle ds by the constriction dg. 

 ks and sx. 1 1- -ad and tail folds of 

 the outer g- i in layer which are 

 beginning tl:e formation of the 

 amnion. rfy.- Yelk duct oropen- 

 iiig by which t he con tents of ds are 

 conveyed to the growing embryo. 

 dd. Intestinal glandular layer 

 forming intest'nal canal, vl. Re- 

 gion of the I.e. irt. 



No. 3. The, head and tail folds 

 <if the ruuiron have approached 

 till they touch, leaving only a thin 

 partiti< n between the spaces on 

 eaeh s'de, am. -Amnion sac. sh. 

 -HSerona membrane. ^/.Intes- 

 tinal fibrous l:iyer. ah. Amnion 

 cavity, a/. Allantois forming at 

 hind end of intestine. 



No. -i. The amnion cavity-aA- 

 nas enlarge i and the serous mem- 

 brane- .sA has been pressed out- 

 ward and consolidated with the 

 prochorion. sz Tufts of sumo. 

 r. Space between the amnion and 

 Chorion filled with fluid. The em- 

 bryo has grown and so 1ms its 

 waste basket, the allantois ai 

 both at the expense of the yelk-sac 

 ds which has correspondingly 

 FIG. L'_'. decreased 



No. 5. The embryo now shows the, gill openings and the rudimentary limbs. The 

 yelk sac OS 18 nearly exhausted. 1 he allantois [<{ in this no. ] has reached the chorion 



and is being attached to it, this part of the latter subsequently becoming the placentv 



ed .up into a hollow cord, which becomes tin 



while the stem of the allantois is shriveled 



nnbi Meal cord having that part of the amnion membrane marked a as its "amnic] 



sheath." (Kdlliki-r.) 



Now to return to development of the egg. 



It consisted (when we left it) of a single layer of cells snn< 



