l.|.\ 1 l.0l'\|i;\ I 01 '1 HI'- I'IMMI : 



167 



nenral tube appears in a C on of a Chick embryo (fig. 107) 



:ts ;i oompad 'I consisting of n, 1 small cells, 



which, as far as it is not divide;! up into separat e blocks, is 

 as primi:: Ql plate 



or protoveitebral plate. In 

 ti-:. 107 it is .-till coin- 

 at the side by means of a 

 thin isthmus of cells with 

 the lateral plates, in whose 

 territory the middle genn- 

 layers are thinner and sepa- 

 rated from each other by a 

 fissure, 



In observing the blasto- 

 "< rm from the surface the 

 region of the primitive-seg- 

 ment plates, as is to he seen 

 in the posterior part of a 

 nine days-old Rabbit embryo 

 108), appears darker than 

 the region of the lateral plate; 

 that the two are dis- 

 tinguished from each other; 

 one is stem-zone (stz), the 

 other parietal zone 



The development of the 

 primitive segments is ob- 

 servable in the ( 'hick at the ^ 

 beginning of the second day 

 of incubation, in the Rabbit 

 at about the eighth day. 

 Clear transverse .streaks ap- 

 pear in the stem-zone at 

 some distance from the primi- 

 tive streak, about in the 

 middle of the embryonic 



fundament, both on the right and the left of the chorda and neural 

 tube (fig. 108). They correspond to transverse lis-ures, by means 

 of which the primitive-segment plates are divided into the small 

 and solid cubical primitive segments (uw). In the nine-days-old 

 Rabbit embryo represented in fig. 108 these plates are resolved in 



