302 THE EVOLUTION OF MAN. 



the central line of the outer surface. On each side of this, 

 the dorsal swellings rise in the form of low ridges. In the 

 centre of the lower side of the primitive groove a cylin- 

 drical band separates itself from the cell-mass of the thick 

 axis-band ; this, which in transverse section appears 

 roundish, is the first rudiment of the notochord (chorda 

 dorsalis, x). The four secondary layers separate more and 

 more distinctly. The intestinal-fibrous layer (f) appears 

 as the product of the intestinal-glandular layer (d), and 

 distinct from the skin-fibrous layer (m), which arises from 

 the skin-sensory layer Qi). 



S/> Ativh d'tl a M ,p '- c i 



Fig. 90. — Transverse section through the germ-shield of an incubated 

 Chick (about the end of the first day) ; about 100 times the natural size. 

 The skin-sensory layer (the outer germ-layer) separates into two different 

 parts : (1) the thinner, peripheric horn-plate Qi), from which the outer skin 

 with its appendages arises ; (2) the thicker, axial spinal plate (m) , which 

 gives rise to the spinal tube (tubus medullaris) ; this originates from the 

 dorsal furrow (Rf), the deepest part of which forms the primitive groove 

 (Pv). The boundaries between the spinal plate (m) and the horn-plate (h) 

 form the prominent, parallel dorsal swellings. The middle germ-layer, the 

 compound fibrous layer (the " motor-germinative "), is already distinguished 

 into the notochord (ch) and the two side-layers (sp). The inner portion of 

 these side-plates soon becomes defined as the primitive vertebral band 

 (uwp). The tiny fissure in the side- plates is the first rudiment of the 

 future body-cavity (uwh). The inner germ-layer (the intestinal-glandular 

 layer) (d d) is not yet modified. (After Kolliker.) 



The primitive groove (Fig. 90 Pv) soon becomes con- 

 siderably deeper and so fashioned as to constitute the bed 

 of the broader spinal furrow (medullary or dorsal furrow) 



