Haddon — Note on the Blastodermic Vesicle of Mammals. 537 



inner cells, the former being smaller than the latter, and tending 

 to enclose them. 



The inner cells next divide, forming thirteen cells in all, and 

 become markedly more opaque and granular : four of these cells 

 pass into the interior of the mass. 



Further subdivision of the outer cells into sixteen cells causes 

 them to extend still further over the inner cells. 



Segmentation continues in both sets of spheres till the tenth 

 stage is arrived at, when the ovum consists of a solid central mass 

 of thirty-two inner cells, entirely surrounded by sixty-four outer 

 cells, except at one spot which van Beneden calls the "blastopore." 

 The outer cells, however, soon entirely surround the inner. 



A cavity makes its appearance between the outer and the inner 

 cells at the opposite pole of the ovum to that at which van Bene- 

 den's blastopore is situated. This cavity rapidly enlarges, and the 

 inner cells depend as a small mass from one pole of the ovum. 



About ninety-four hours after fertilization the ovum measured 

 0'28mm., the diameter having tripled within the twenty- four hours 

 since the ovum entered the uterus. It now consists of a thin walled 

 vesicle, the blastodermic vesicle, the wall of which consists of a single 

 layer of flattened cells, the products of the outer spheres. At one 

 pole the inner cells form a flattened disc, the centre of which is two 

 or three cells thick ; the periphery is constituted of several rounded 

 and more or less completely isolated cells. Later, these two por- 

 tions of the inner mass become more distinguishable ; the more co- 

 herent central portion initiates the circular embryonic area (area 

 pellucida), while the irregular outer zone is formed of a single 

 layer of amoeboid cells, and gradually extends over the inner sur- 

 face of the blastodermic vesicle, constituting the area opaca. 



During the fifth day the blastoderm consists of three layers, 

 which van Beneden regards as the three embryonic layers. He 

 tabulates them thus (p. 179) : — 



Stage V. (102 hours, or four days, six hours). 



Stage YI. (124 hours, or five days, four hours). 



V. VI. 



Ectoderm = Epihlast 

 Mesohlast 

 + Hypoblast. 

 In other words, the diploblastic fifth stage is succeeded by a tri- 



Endoderm = \ 



