THE HUMAN EMBRYO. 375 



three higher vertebrate classes, while the third, the yelk- 

 sac, occurs in most Vertebrates. This circumstance is very 

 significant, and we shall afterwards find that it affords 

 material assistance towards the construction of the 

 genealogical tree of Man. 



The nature of the outer egg-membrane, which surrounds 

 the entire egg embedded in the uterus of the Mammal, is 

 the same in Man as in the higher Mammals. At first the 

 egg is surrounded, as we have already stated, by the trans- 

 parent, structureless zona pellucida (Fig. 1, p. 122, and Fig. 

 36-40, pp. 210-212). Very soon, however, even in the first 

 week of development, its place is taken by the permanent 

 tufted membrane (choriori). This originates from the outer 

 fold of the amnion, from the so-called serous membrane, 

 the formation of which we shall presently examine. It is 

 formed of a single stratum of cells from the outer germ- 

 layer, the skin-sensory layer. At its first appearance the 

 serous membrane is an entirely simple, flat, closed vesicle ; 

 like a wide sac, closed in all directions, it surrounds the 

 embryo with its appendages; the intermediate space be- 

 tween the two is filled with clear watery fluid. At an 

 early period, however, the smooth outer surface of the sac 

 becomes covered with numerous small tufts or knots, which 

 are really hollow processes, resembling the fingers of a glove 

 (Fig. 127 ; 139, sz, 5 chz). These branch and grow into 

 the corresponding depressions formed by the bag-like glands 

 of the mucous membrane of the maternal uterus ; the egg 

 thus acquires its permanent, fixed position (Figs. 130, 132, 

 134). 



In the human egg, even between the thirteenth and 

 fourteenth day, this outer egg-membrane, which we shall 



