Embryology. 15 



darker internal 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 are 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, m. Middle 

 germ layer, developed from the 

 other two. kh. Intestinal germ 

 vesicle or yelk. d Prochorion 

 with its tufts d. 



No. 2. c. The embryo begin- 

 ning to be separated from the germ 

 vesicle ds by the constriction dg. 

 ks and ss. Head and tail folds of 

 the outer germ layer which are 

 beginning the formation of the 

 amnion. dg. Yelk duct or open- 

 ing by which the contents of ds are 

 conveyed to the growing embryo. 

 dd. Intestinal glandular layer- 

 forming intestinal canal, vl. Re- 

 gion of the heart. 



No. 3. The head and tail folds 

 of the amnion have approached 

 till they touch, leaving only a thin 

 partition between the spaces on 

 each side, am. Amnion sac. sh. 

 Serous membrane, df. Intes- 

 tinal fibrous layer, ah. Amnion 

 cavity, al Allantois forming at 

 hind end of intestine. 



No. 4. The amnion cavity-aA- 

 has enlarged and the serous mem- 

 brane sh has been pressed out- 

 ward and consolidated with the 

 prochorion. sz. Tufts of same. 

 r. Space between the amnion and 

 chorion filled with fluid. The em- 

 bryo has grown and so has its 

 waste basket, the allantois al 

 both at the expense of the yelk-sac 

 ds which has correspondingly 

 decreased. 



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

 yelk sac-rds is nearly exhausted. The allantois [_cl in this no. ] has reached the chorion 

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

 while the stem of the allantois is shriveled up into a hollow cord, which becomes the 

 tinbilical cord having that part of the amnion membrane marked as as its "amnion 

 sheath." (Kolliker.) 



Now to return to development of the egg. 



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



FIG. 22. 



