DKrEI.iiI'MKXr OF THE KMIinVO. 71 



SECTION I.— DEVKLOPMENT OF THE EMBRYO. 



The ovum having been lodged in the uterus, and the f/erm-mcmhraTie 

 or blastoderm having divided into two layers — an upper or seroics, and a 

 lower or mucous — and between which, at a later period, a vascular layer 

 is developed, another modification occurs by which the outline of the 

 embryo becomes evident. 



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 jtrolujera or pcllucula, in the centre of which, and in the trans- 

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

 connected cells in a small rounded mass, which forms the rjerminative 

 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." 



Fig. 42. 



Blastodkbm and Pkimitivk Thack. 



a. Vitelline Membrane with iti) <X)iiiiiiencing VilloBities ; h, External (or Serous) 

 Layer of the Bl*«t<xierui ; r. Internal (or Mucouh) Layer ; d, liody of the 

 Embryo ; //, b'. Earliest Cephalic and Caudal Elevation of the External 

 Layer. 



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

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

 becoming 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 

 constricting in its middle part, is named the area vasculosa or einhnjonic. 

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

 furrow, duo to changes occurring in the primitive streak ; and below it a 

 round 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 

 blastoderm, at the points corresponding to the embryo, becomes doubled, 

 so as to form a new layer — the middle layer. The chorda dorsalis 



