THE PRIMITIVE STREAK. 299 



miment ; this is the " primitive streak " of Baer, the " axial 

 plate " of Kemak. This phenomenon is due to the fact that 

 the upper and middle germ-layers coalesce along their 

 central lines, thus forming the axis-band at this point. 

 (Cf. Fig. 88, 89.) In the centre of the primitive streak an 

 even, dark line, the so-called primitive groove, becomes 

 denned (Fig. 84, 85, a). This separates the germ-shield into 

 two symmetrical halves, a right and a left half. While the 

 primitive groove deepens, the oval germ-area (a. germina- 

 tiva) resumes its earlier circular form. 



The germ-shield, on the other hand, leaves its oval 

 form and assumes the so-called lyre-shape, or sole-shape. 

 Its elliptical leaf-shaped body becomes somewhat pinched 

 in the middle, while its anterior and posterior ends become 

 somewhat enlarged (Fig. 85). This very characteristic 

 shape, which is most aptly compared to the sole of a shoe, 

 a violin, or a lyre, is retained for some time by the embryo 

 of the Mammal (Fig. 86, 87), and also by that of the Bird 

 and the Keptile. The human germ-shield assumes this sole- 

 form as early as the second week of its development. 

 Towards the end of that week its length is about two 

 millimetres. 



We will now leave the peripheric part of the germ- 

 area, for its changes are only interesting to us at a much 

 later period, and we will give our whole attention to 

 the sole-shaped germ-shield, from which the further evolu- 

 tion of the body directly proceeds. In order correctly to 

 understand this, we must employ a method which was first 

 turned to full account by Kemak, viz., that of viewing 

 sections made from right to left perpendicularly through the 



thin disc of the germ-shield. It is only by very carefully 

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