298 



EMBBYOLOGY. 



A, B, d). Somewhat earlier than this, namely, about the 

 fourteenth hour, the first rudiments of the embryo become 

 distinctly visible in the middle of the germinal area, in the 

 guise of a delicate white elongated streak, about a line and a 

 half in length ; it is designated nota primitiva the primitive 

 streak, and lies in the line of the long axis of the germinal 

 area, which itself lies in the transverse axisof the egg (fig. 325, a). 

 Under the nota primitiva, the cumulus proligerus, deeply seated, 

 may still be seen very plainly glistening through (fig. 326, A, 



B, d). The nota primitiva rises slightly above the level of 

 the germinal area (fig. 326, b) ; it is thicker and blunter ante- 

 riorly, or towards that end which becomes the head of the 

 embryo, thinner, and tending to a point posteriorly. The nota 

 primitiva is probably the groundwork of the brain and spinal 

 cord. 



[ 484. The nota primitiva, an aggregate of dark granules 



in the first instance, becomes 

 more fluent by and by, and 

 presents itself as a layer of de- 

 licate, transparent masses, by 

 the side of which, between the 

 sixteenth and eighteenth hour, 

 a pair of new formations arise 

 symmetrically, near the mid- 

 dle line. These are the lami- 

 nce s. plicce dorsales the dor- 

 sal laminae, two cylindrical 

 rolls or enlargements, which 



Fig. 326. Ideal sections of fig. 

 323 (after Baer, with slight varia- 

 tions). A, transverse section ; B, 

 longitudinal section ; a, vitelline 

 membrane, indicated by a finely 

 dotted line ; b, nota, or primitive 

 streak, with the serous layer of the 

 blastoderm a, corresponding to the 

 area pellucida ; c, mucous layer of 

 the blastoderma, corresponding to 

 the area vasculosa ; rf, cumulus pro- 

 ligerus s. nucleus cicatriculae. 



arise parallel to the nota primi- 

 tiva, and form a couple of cris- 

 tae, or ridges, one on either 

 side of it (figs. 327 and 328, 

 b, b), which diverge anteriorly 

 and posteriorly, being nearest 

 about the middle of their 

 length, and sloping somewhat 

 from without inwards, or to- 



wards one another. The angles of the ridges are softly 

 rounded off; each ridge has the appearance of a clear broad 

 line, which is included within two darker lines. The germinal 



