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CHANGES IN THE OVUM. 67 



In the centre of the blastoderm, where it is supposed to divide into 

 these layers, there is observed a clear space which has been designated 

 the area proligera or pellimda'vcv the centre of which, and in the trans- 

 verse axis of the vitellus, there is going on a multiplication of loosely con- 

 nected cells in a small rounded mass, which forms the germinative area, 

 primitive streak, or trace of Von Baer ; while around this another space — 

 the area vasculosa — is developed. The '' primitive streak " is the centre 

 where the blastoderm commences to be separated into the external and 

 internal layers ; the middle layer, which is to form the foetal organs, not 

 extending beyond the "germinative area " or "streak." 



By a proliferation of the elements of the outer and middle layers of the 

 blastoderm, the primitive trace increases in surface and thickness, be- 

 coming clearly defined and prominent, in the form of a shield, and is 

 named the opaque area, which, when it grows transparent in its centre, is 

 named the transparent or pellucid area ; the clear spot, narrowing or con- 

 stricting in its middle part, is named the area vasculosa or embryojiic. In 

 the middle of this embryonic area appears a dark line, or median fur- 

 row, due to changes occurring in the primitive streak ; and below it a 

 sound cord — the chorda dorsalis — the axis of the future embryo, and 

 which is to develop the spine ; on each side of the streak is the lamina 

 dorsalis, and the portion of fluid separating them from the chorda dor- 

 salis is the future spinal cord and brain. The inner layer of the blas- 

 toderm, at the points corresponding to the embryo, becomes doubled, so 

 as to form a new layer — the middlle layer. The chorda dorsalis thickens 

 at the front part, to form the first appearance of the skull, and the fluid 

 between the dorsal laminae is in larger quantity, in correspondence with 

 it ; so that the central parts of the nervous system and their coverings 

 are laid down at the same time and grow simultaneously. The separa- 

 tion between the spinal, cord and brain takes place early, being coinci- 

 dent with a curving downwards, towards the yolk, of the anterior part of 

 the laminae dorsalis, which defines the limit between the skull and spine, 

 brain and cord. 



Next follows the closing of the dorsal laminae over the fluid which is 

 to constitute the brain and cord. Two other laminae are in the mean 

 time proceeding from the axis of the embryo, one on each side. They 

 grow out laterally, and tend to converge in the median line, as did the 

 dorsal laminae ; but they form a larger curve, and follow a different 

 direction, converging to meet below the axis, where they join, except at 

 the umbilicus. 



After the rudiments of organic life have been commenced in the cen- 

 tral portion of the serous layer, a fold of its peripheral portion arches 

 over the dorsal surface of the embr}'0, so as to represent a sack whose 

 opening is at the edge of the fold. The opening gradually decreases 

 until the opposing folds of membrane are in contact ; it then disappears, 

 leaving the foetus surrounded by two membranes. The one next the 

 foetus is the amnion, and the other is gradually separated from the amnion 

 and joins the serous lamina of the blastoderm, forming the "false 

 amnion" of Pander, or the " serous covering " of Von Baer. The mem- 

 brane surrounding the vitellus or yoke is very vascular ; it becomes oval 

 in shape, and more pointed when it is in contact with the embr3'o, until 

 at length it contracts into a narrow duct, constituting the vesicula alba 

 and duct. Thus, then, we have seen the embryo developed in the layers 

 of the blastoderm, and formed by a gradual closing in of the laminae to- 



