168 



EMBRYOLOGY. 



front into eight pairs of primitive segments (uw), whereas in the 

 hind end of the embryonic area they still have the form of a con- 

 tinuous mass of cells, the 

 stem-zone (sfe), which in sur- 

 face-views appears darker 

 than its surroundings. 



In a somewhat more ad^ 

 vanced stage the primitive- 

 segment, which probably se- 

 cretes at the same time fluid, 

 develops in its interior, as 

 in the case of the Amphibia 

 and Selachii, a cavity, around 

 which the cells group them- 

 selves in a radial manner. 

 This cavity, too, is at first in 

 communication laterally with 

 the fissure of the body-cavity, 

 until the primitive segment 

 has been fully constricted 

 ofi-. 



In Vertebrates, besides the 

 trunk-region, a part of the 

 head-region of the embryo is 

 also affected by this process 

 of segmentation which we 

 have been considering. Wa 

 must therefore speak in the 

 one case of head-segments, 

 and in the other of trunk- 

 segments. Up to the present 

 time the number and condi- 

 tion of the head-segments have 

 been made out (by Balfour, 

 MiLNES Marshall, and van 

 Wijhe) most accurately for 



Fi^. 108.— Rabbit embryo of the ninth day, seen 

 from the dorsal side, after Eolliker. Magnified 

 21 diameters. 



The stem-zone (stz) and the parietal zone (pzj are 

 to be distinguished. In the former 8 paira of 

 primitive segments have been established at the 

 side of the chorda and neural tube. 



ajp, Area pellucida ; vf, medullary groove ; vhj fore 

 brain ; ab, eye-vesicle ; mh, mid brain ; M, hind 

 brain ; «w, primitive segment ; stz, stem-zone ; 

 pz, parietal zone ; h, heart ; p\ pericardial part 

 of the body-cavity ; vd, margin of the entrance to 

 the head-gut (I'ordere Bamvpfort^, seen through 

 the overlying structures ; «/, amniotic fold ; vo, 

 vena omphalomesenterica. 



the Selachians. In this in- 

 stance there are nine pairs of hollow head-segments. In the higher 

 Vertebrates such segments although fewer in number, have also 

 been described; however, the less sharply differentiated structures 

 of the latter demand stUl further investigation. 



