450 



EMBRYOLOGY. 



and downward, inserts itself between the 

 investing primitive epidermis. In this way 

 angle of the primitive somites (us), which 



tpg 



tube and the closely 

 it reaches the dorsal 

 are by this time well 

 developed. Then the 

 neural crest, as BAL- 

 FOUR names it, or the 

 yanglionic ridge, as 

 SAGEMEHL calls it, 

 is divided up into 

 successive regions. 

 For the tracts which 

 alternate with the 

 primitive segments 

 lag behind in then- 

 growth, while the 

 parts lying opposite 

 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. Inas- 

 much as the longi- 

 tudinal axis of the 

 Lizard embryo em- 

 ployed for the sec- 

 tions was greatly curved, the five segments seen in the section are cut 

 at 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-vessels 



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

 after RABL. 



'i he 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, sk, from which the 

 skeletogenous tissue is developed, ch, Chorda ; spg, spinal 

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

 tch, subchordal rod ; ao, aorta ; ik, inner germ-layer ; 

 pmb, parietal, vmb, visceral middle layer. 



K, Cross Motion through a Lizard embryo, after SAGEMEHL. 



rm, Spinal cord ; spy, lower thickened part of the neural 

 ridge ; s^tf, its upper attenuated part, which is continuous 

 with the roof of the neural tube ; t, primitive segment. 



