ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 759 



account, partly iu consequence of the development of the ectoderm, 

 appears thinner than it really is. Some of them lie scattered in the 

 germinal cavity ; others rest on its floor. 



The first consequence of incubation, apart from the increase of all 

 the cells of the germ, is the transformation and union of its deepest 

 residual cells to constitute the monotleric inner lamina or " endo- 

 blastoderma." This layer consists of cells no longer rounded hut like 

 those of an epithelium and flattened, save where they are rendered 

 thick by their large nuoleus, so that in sections they seem fusiform. 

 The under surface of the whole germ is protected by the inner lamina, 

 which therefore roofs in the germinal cavity and beyond the latter is 

 in close contact with the white yolk. Whether this outer portion be 

 first formed or the transformation of residual into endodermic cells 

 proceed contrary-wise, from centre to circumference, has not been 

 determined. 



In a bird's-eye view of the germ at this stage are seen an area 

 pellucida and an area opaca. The former is conterminous with 

 the germinal cavity. The latter corresponds to the peripheric region 

 of the germ, resting on the surface of the white yolk. Such a germ 

 differs from a typical gastrula in that between its outer and inner 

 laminae are included cells not derived from either. These are the 

 cells of the " middle-germ," or mesoblast. 



Soon after the inner lamina can be distinguished, a dark spot shows 

 itself, somewhat excentrically, in the clear area. This is the em- 

 bryonal shield of von Baer. It is mainly due to a thickening of the 

 outer lamina, and its appearance marks the development of the 

 primitive streak, whose broadened cephalic border it represents. The 

 primitive streak owes its origin to an invagination of a part of 

 the outer lamina. The part so invaginated is cut off. But it soon 

 ceases to form one whole clearly separable from the cells of the middle- 

 germ, wherein it becomes implanted. For here cells are associated 

 which have a twofold origin, ectodermic and mesoblastic. Hence 

 many of the contradictions into which discordant observers have 

 fallen. [It is a pity that time did not permit Dr. Wolff to study 

 the remarkable conclusions of the late Prof. Balfour on this subject.*] 

 The floor of the primitive streak, thus reinforced by a copious con- 

 tribution of cells from the middle-germ, Dr. Wolff terms the " axial 

 plate." As the germ continues to grow, the axial plate gives rise to 

 the rudiment of the cerebro-spinal system, the primitive vertebrae 

 with the lateral plates, and the chorda. The primitive groove and 

 its boundaries are designated, in consequence, dorsal. Our author 

 does not consider the medullary and primitive grooves as distinct 

 formations, and denies the possibility of a displacement of one in front 

 of the other. The peripheral portion of the axial plate extends into 

 the opaque area. To this portion, chiefly, if not exclusively, derived 

 from cells of the middle-germ, the phase *' vascular lamina " may con- 

 veniently be applied. 



The accumulation of cells from the middle-germ about the ecto- 

 dermic rudiment of the primitive streak is contemporaneous with 

 * See Quart. Journ. Micr. Sci., xxii. (1882), p. 174, and this Journal, ante, p. SH. 



