408 - THE EVOLCTTION OF MAN. 



referring it to the horn-plate, others to the primitive ver- 

 tebral plate, and yet others to the skin-muscle plate. Pro- 

 bably its earliest (phylogenetic) origin is to be found in the 

 skin-sensory layer ; but it very soon quits its superficial 



ch u>r ao sp <^<^ '^f 



Fig. 318. — Transverse section through the embryo of a Chick, on the 

 second day of incubation : h, horn-plate ; m, medullary tube ; ung, primitive 

 kidney duct ; ch, notochord ; mv, primitive vertebral cord ; h'pl, skin- 

 fibrous layer ; df, intestinal-fibrous layer ; m/p, mesentery-plate, or middle 

 plate (point of attachment of the two fibrous layers) ; sp, body-cavity 

 (cosloma) ; ao, primitive aorta ; dd, intestinal-glandular layer. (After 

 Kolliker.) 



position, passes inward, between the primitive vertebral 

 plates and the side plates, and finally lies upon the inner 

 surface of the body-cavity. (Cf Figs. 66-69, u, vol. i. p. 277, 

 ^ and Figs. 95-98, p. 319; also Plate IV. Figs. 3-6, u.) While 

 the primitive kidney duct is thus making its way inward, 

 on its inner and under side appear a large number of small 

 horizontal tubes (Fig. 319, a), exactly corresponding to the 

 segmental canals of the Myxinoides (Fig. 317, h). Like the 

 latter, these are, probably, originally protuberances of the 

 primitive kidney ducts (Fig. 316, t^). At the blind, inner 

 end of each of the primitive urinary tubes an arterial 

 glomerulus is formed, which grows into this blind end 

 from within, forming a " vascular coil." The glomerulus 

 t(j a certain extent expands the bladder-like blind end 

 of the small urinary tubes. As the primitive urinary tubes, 



