THE BLASTODERMIC VESICLE. 



355 



completely round the larger inner cells, which from this time 

 they surround on all sides. 



The outer layer of cells now begins to grow rapidly ; the 

 central or inner cells remaining attached to the outer layer at one 

 spot, but becoming separated from it at all other parts. By about 

 the seventy-fifth hour, i.e. four or five hours after entering the 

 uterus, the ovum has acquired the structure shown in Fig. 139 : 

 the outer layer of cells, CC, forms a hollow ball, about 0'12 mm. 

 in diameter, to the inner surface of which the mass of inner cells. 



Fig. 140. — Section of the blastodermic vessel of a Babbit at the end of the 

 fourth day. (After Van Beneden.) x 250. 



CC, outer layer of cells. CD, inner lenticular mass of cells. CV, cavity of the 

 blastodermic vesicle. 



CD, is attached at one spot, the rest of the cavity of the ball, cv, 

 between the outer and inner cells, being filled with fluid. 



The growth of the ball, or blastodermic vesicle, as it is now 

 termed, proceeds rapidly ; and by the end of the fourth day, i.e. 

 about twenty-four hours after entering the uterus, the structure 

 and proportions are as represented in Pig. 140. The vesicle is 

 still spherical, measuring on an average about 0-28 mm. in 

 diameter. It consists of an outer wall of flattened polygonal 



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