DEVELOPMENT OF THE MAMMALIAN EMBRYO. 



part. The comparative obscurity of the outer space the area vasculosa 

 is due to the greater accumulation of nucleated cells and nuclei at that part 

 than in the area pellucida. The first trace of the embryo in the centre of 

 the area pellucida, consists, not of a projecting line, as described by Von Baer, 

 but of a shallow groove (E), as shewn especially by Reichert in the embryo 

 of the chick.* This is formed in the external or serous fold alone of the ger- 

 minal membrane: the mucous fold having no direct share in its production. 

 Coincidently with the formation of the primitive groove, two oval masses, 

 the laminae dorsales (D) appear, one on each side of the groove. Their 

 form changes according as does that of the area pellucida : passing gradually 

 with the latter from an oval to a pyriform shape, and eventually becoming 

 guitar-shaped. The upper borders of these two masses gradually tend to- 

 wards each other, as in the embryo of the chick, and ultimately unite, so 

 as to convert the primitive groove into a canal or tube. But with regard 

 to the mode in which these masses unite, and to their own nature, and the 

 changes which ensue in them, Bischoff maintains a different view to that 

 advocated by Reichert. The latter physiologist supposed that, at least in the 

 chick and in the frog, these oval masses constitute the rudimentary parts 

 of the central nervous system, but Bischoff is of opinion with Von Baer, 

 that in Mammalia they are the foundations of the dorsal part of the trunk 



of the embryo, while the 

 nervous system is deve- 

 loped, as will be hereafter 

 more fully described, only 

 from their most internal 

 part, that, namely, which 

 forms the bottom and 

 sides of the primitive 

 groove. Shortly after, 

 or, as shewn in fig. 11 

 from a bitch's ovum, even 

 before the laminse dorsa- 

 les have closed over the 

 primitive groove, a few 

 square-shaped, at first in- 

 distinct, plates, which are 

 the rudiments of vertebrae 

 (fig. 1 1, D), begin to ap- 

 pear at about the middle 

 of each. It is not possible to perceive at this time the chorda dorsalis 

 so distinctly observed in birds and fish ; but later, when the bodies of the 



* Muller's Physiology, p. 1547. 



t Fig. 1 1 . Portion of the germinal membrane, with rudiments of the embryo from the 

 ovum of a bitch. The primitive groove, A, is not yet closed, and at its upper or cephalic 



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