450 



EMBRYOLOGY. 



and downward, inserts itself between the tube and the closely 

 investing primitive epidermis. In this way it reaches the dorsal 

 angle of the primitive somites (us), which are by this time well 



developed. Then the 

 neural crest, as BAL- 

 FOUB names it, or the 

 f/anglionic ridge, as 

 SAGEMEHL calls it, 

 is divided up into 

 successive regions. 

 For the tracts which 

 alternate with the 

 primitive segments 

 lag behind in their 

 growth, while the 

 parts lying opp<ite 

 the middle of seg- 

 ments grow more 

 vigorously, become 

 thickened, and at 

 the same time ad- 

 vance farther ven- 

 trad, penetrating be- 

 tween primitive seg- 

 ment and neural 

 tube. 



Frontal sections 

 furnish very instruc- 

 tive views of this 

 stage. Fig. 259 ex- 

 hibits such a section, 

 taken from SAGE- 

 MEHL'S work. In.-i.-- 

 much as the longi- 

 tudinal axis of the 

 Lizard embryo -m- 

 ployed for the sec- 

 tions was greatly curved, the five segments seen in the section ar 

 t different heights, the middle one deeper than the two preceding 

 and the two following. In the middle segment the fundament of 

 the ganglion (spk) is differentiated and it is bounded by blood-V( 



Fig. 258.- A, Grots section through an embryo of Pristiurus, 

 after RAUL. 



'Jlie primitive segments are still connected with the remaining 

 portion of the middle germ-layer. At the region of tran- 

 sition there is to be seen an outfolding, , from which the 

 akeleto {enoiis tissue is developed, cfi, Chorea ; spg, spinal 

 ganglion ; mp, muscle-plate of the primitive segment ; 

 <cA, subchordal rod ; no, aorta ; ik, inner germ-layer ; 

 pmb, parietal, vmb, visceral middle layer. 



J>. Cross section through a Lizard embryo, after SAGKMIIII . 



-<iu, Spinal cord ; spy, lower thickened part of the neural 

 ridge ; ptf, its upper attenuated part, which is continuous 

 witli the roof of the neural tube ; us, primitive segment. 



