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 (stz), which in sur- 



.^aiiWBa^^ 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 

 off. 



In Vertebrates, besides the- 

 trunk-region, a part of tho 

 head-region of the embryo is 

 also affected by this process, 

 of segmentation which we- 

 have been considering. We 

 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 

 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 ulso 

 been described; however, the less sharply differentiated structuies 

 of the latter demand still further investigation. 



Fig. 108. Rabbit embryo of the ninth day, seen 

 from the dorsal side, after KOLLIKER. Magnified 

 21 diameters. 



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

 to be distinguished. In the former 8 pairs of 

 primitive segments have been established at the 

 side of the chorda and neural tube. 



ap, Area pellucida ; rf, medullary groove ; vh, fore 

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

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

 pz, parietal zone ; h, heart ; ph, pericardial part 

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

 the head-gut (vordere Darmpforie), seen through 

 the overlying structures ; of, amniotic fold ; vo, 

 vena omphalomesenterica. 



