82 FORMATION OF THE AREA GERM1NAT1VA. 



in the bitch by the zona pellucida alone, is textureless, solid, but exceed- 

 ingly fine and delicate. The internal vesicle, on the other hand, is 

 formed, in the manner before described, of hexagonal cells ; within each 

 of which a nucleus is usually observed. This internal vesicle, or vesi- 

 cula blastodermica, shortly after it is formed, undergoes a rapid increase 

 in extent and thickness; its growth being effected apparently by the 

 development of new cells. Concerning the mode in which these new 

 cells are developed, however, Bischoff cannot speak with certainty, though 

 having observed that while the growth of the vesicula blastodermica 

 proceeds, the granules within the already formed cells gradually diminish 

 in number, he thinks it not improbable that these granules are employed 

 in the production of new cells, and that possibly each granule constitutes 

 a nucleus, around which a fresh cell is developed;* but he has never 

 witnessed anything resembling the development of one cell within another, 

 and he considers it very questionable if this mode of multiplication is 

 ever pursued in the yolk. 



Very soon after its formation, the vesicula blastodermica presents at 

 one point on its surface an opake roundish spot, which is produced by 

 an accumulation of cells and nuclei of cells, of less transparency than 

 elsewhere. This space, the " area germinativa " (Fruchthof ), is the part 

 at which the embryo first appears. It was supposed by Reichert,f that 

 in the chick, and also in Mammalia, the appearance of this, the first 

 trace of the embryo, was preceded by the formation, at the surface of 

 the yolk, of an investing membrane, beneath which the area germinativa 

 is formed, but Bischoff shews that certainly in Mammalia (as Vogt also 

 shewed to be the case in amphibia and fishes, page 80), no such invest- 

 ing membrane exists, and that the area germinativa is formed upon the 

 surface of the germinal membrane and covered only by the zona pellucida, 

 or external membrane of the ovum as it has now become.} 



Bischoff has also found that the germinal membrane of the Mammi- 

 ferous ovum presently becomes divided into two distinct lamime, in the 

 same manner as has been long known to take place in the ova of birds. || 

 This division is at first most manifest at the situation of the area ger- 

 minativa, but it soon extends from this point and implicates nearly the 

 whole of the germinal membrane. Bischoff has adopted for these lamina 

 the same names that are applied to them in the chick, namely, for the 

 external one, the serous, or animal layer ; for the internal one, the mucous, 

 or vegetative layer. He has not been able to find the third layer 

 described by Reichert as existing in the ova of the chick, and called by 

 him membrana intermedia.^ 



* Entwickelungs-geschicte des Hunde-eies, p. 66. t Muller's Physiology, p. 1543. 

 Entwickelungs-geschichte des Hunde-eies. p. 68. 



$ Entwickelungs-geschichte der Saugethiere und des Menschen. Leipsic, 1842, p. 59. 

 || Mviller's Physiology, p. 1533. Tf Miiller's Physiology, p 1544. 



