682 THE OVOI AND BLASTODEEM. 



formation, and before any incubation has taken place, already consists 

 of two layers of cellular elements (fig'. 496, s and d). 



These two layers diflfer considerably. The cells of the upper layer 

 are of smaller diameter, about a-^Vo"? more compactly laid together, so 

 as to be slightly compressed, and shortly prismatic, and are all provided 

 with distinct nuclei. Those of the lower layer are of somewhat larger 

 size, and of a more granular aspect, so as to hide tlie nucleus, which 

 appears, however, to exist in the greater number, and the whole of 

 tliese cells are rather scattered in reticular groups than united into a 

 distinct and consistent layer (His). Below this layer there is a narrow 

 space occupied by clear fluid between the germ and the surface of the 

 white yolk, to which the name of sulnjerminal cavity is given, and in 

 this space a number of granular spheres or formative cells are found, 

 somewhat similar to the cells of the lower layer. 



Fig. 496. 



Fig. 496. — JMiciioscopic view of a vertical section thkouoh half the Blasto- 

 derm OF A newly-laid Egg. (From Strieker), -f" 



S, upper layer of small nucleated cells ; D, lower layer of larger gramilar cells ; M, 

 segment spherules lying in t!ie subgermiual cavity ; A, substance of tlie white yolk 

 below the germ. 



In mammals, too, it would appear from the observations of Bischoff, 

 Coste, Reichert, and others, that the blastoderm which covers the yolk 

 after the completion of segmentation, though not double from the 

 first, comes soon to consist of two layers. The exact time and mode of 

 the appearance of a second layer are, however, still imperfectly known : 

 and, from the difficulty belonging to the question of secondary seg- 

 mentation in the deeper part of the yolk previously adverted to, it 

 may be doubtful how far the whole blastoderm of mammals is to be 

 regarded as the direct product of a primary segmentation, or a part 

 of it is due to a later organising process. 



There is, however, a great difference in the relation of the primi- 

 tive blastoderm to tlie rest of the ovum in birds and in mammals. In the 

 former, as already stated, previous to incubation, this organised cellular 

 disc cover.-=; only a very limited part of the surface of the yolk, while 

 in mammals it completely surrounds the yolk from the first, and thus 

 constitutes thQvesimlar blastoderm of Coste, Keichert, and other authors. 



From tlie first the blastodermal disc of birds shows a difference in 

 its centra] and peripheral parts, the former being thinner and more 

 transparent, and thus forming the so-called transjjarent area, the latter 

 being thicker and more opaque, is the opaque area. But in mammals 

 the central portion of the primitive blastoderm presents no defined 

 transparent area, and differs chiefly at first from the rest by its greater 

 thickness, and it is by later changes accomjmnying development that 

 there arises a thickened opaque disc, the eml^rijonal spot of Coste, and 



