192 Mr. W, Heape. On the Germinal Layers [Dec. 22, 



nation of which leads me, in the main, to agree with Lieberkiihn's 

 aceonnt of the development of the embryonic layers of that animal. 



I have not been able to follow completely the conrse of segmenta- 

 tion, nor have I been able to trace a differentiation of the segments 

 into two layers, an outer and an inner, thongh there appears to be no 

 doubt, on aceonnt of the arrangement of the spheres in a somewhat 

 later stage, that Van Beneden's description of a fnlly segmented 

 ovum is substantially correct. 



The earliest specimen of an ovnm in my possession after the com- 

 pletion 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, elsewhere 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 millim., and that of the inner 

 mass of cells "06 millim. 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 millim. diameter, the diameter of the inner mass, 

 which is of an approximately spherical shape, is less, being respec- 

 tively "04, "04, '0-4, '05 millim., than in the youngest vesicle, mea- 

 suring as stated above, Tl millim. in which the inner mass is 

 "06 millim. diameter. 



Sections through a vesicle measuring '25 millim. diameter (the 

 inner mass measuring about '04 millim.) 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 embryonic area becomes 

 enlarged. 



In a vesicle "44 millim. diameter, the inner mass of cells is seen to 

 be commencing to divide into two layers, and in a vesicle "57 millim. 

 in diameter, in which the inner mass is "08 millim. in diameter, 

 this division is completed, and a layer composed of a single row of 



