GERMINAL LAYERS AND EARLY DEVELOPMENT OP MOLE. 109 



The earliest specimen of an ovum in my possession after 

 the completion of segmentation is similar to that figured in 

 Van Beneden^s paper (loc. cit.), Plate iv, fig. 6, iii, and in 

 Lieberkiihn's paper (loc. cit.), fig. 1. 



The ovum consists of an outer layer and an inner mass of 

 cells, between and partly separating which is a cavity. The 

 outer layer has the form of a sphere of somewhat flattened 

 cells, while the inner mass is composed of irregularly polygonal 

 cells ; these two are attached together for a small area, else- 

 where they are separated by a cavity, the blastodermic cavity, 

 which is seen in optical section as a crescent-shaped space 

 partially surrounding the inner mass of cells. The diameter 

 of this ovum measures '11 mm., and that of the inner mass 

 of cells '06 mm. A thick zona invests the ovum. 



Upon the formation of the blastodermic cavity the ovum 

 may be called the blastodermic vesicle. 



The vesicle becomes enlarged, and I am inclined to 

 believe that during the enlargement the cells of the inner 

 mass assist in the formation of the outer wall of the vesicle, 

 since in various vesicles of about '2, '25, '3, '38, mm. diam- 

 eter, the diameter of the inner mass, which is of an approx- 

 imately spherical shape, is less, being respectively "04, '04, 

 •04, '05 mm., than in the youngest vesicle, measuring as 

 stated above, '11 mm. in which the inner mass is '06 mm. 

 diameter. 



Sections through a vesicle measuring '25 mm. diameter 

 (the inner mass measuring about -04 mm.) show the outer 

 layer to be composed of greatly flattened cells closely 

 applied to the zona, which is now much thinner, owing to 

 the expansion of the vesicle, while the inner mass in the 

 form of a solid mass of irregularly rounded cells is attached 

 to the outer layer for a small circular region, which I shall 

 speak of as the embryonic area. 



As the vesicle enlarges the inner mass of cells slightly 

 flattens out and widens at the same time, so that the embry- 

 onic area becomes enlarged. 



In a vesicle '44 mm. diameter the inner mass of cells is 

 seen to be commencing to divide into two layers, and in a 

 vesicle '67 mm. in diameter, in which the inner mass is "08 

 mm. in diameter, this division is completed, and a layer 

 composed of a single row of slightly flattened cells is sepa- 

 rated off from and underlies the main portion of the inner 

 mass of cells ; this layer is the hypoblast. The main 

 portion of the inner mass of cells is undergoing at the same 

 time a change in structure, inasmuch as some of the polyg- 



