484 



APPENDIX 



have the complete failure of the nervous system to differentiate in 

 the ectodermal portion. The only difference observable between 

 presumptive epidermis and presumptive neural plate is that the 

 latter shows an autonomous tendency to elongation in the direction 

 of the egg's major axis. We shall later return to the absence of 

 ectodermal differentiations. 



In marked contrast with this incapacity of the ectoderm is the 

 capacity of the endo-mesoderm for self-differentiation. We have in 



^^^ 



Fig. 216 



Transverse section through trunk-region of exogastrulated endo-mesoderm, 

 axolotl. Note superficial endoderm surrounding notochord, myotomes, con- 

 nective tissue, and (below on the right) heart. (From Holtfreter, Arch. Entwmech. 

 cxxix, 1933.) 



the first place typical notochord. Then the mesoderm produces the 

 following derivatives: in the anterior region, head-musculature, 

 mesenchyme and cartilage ; in the trunk-region, somitic mesoderm, 

 pro- and meso-nephric tubules with coelomic funnels and asso- 

 ciated with gonads, smooth gut-musculature and (empty) hearts 

 capable of rhythmical contraction, empty endothelial sacs, masses of 

 blood-cells, coelomic spaces, and connective tissue. The endoderm 

 shows equal powers of self-differentiation, and gives rise to buccal 

 cavity, pharynx with endodermal portion of the gill-clefts (the 

 visible apertures on the surface of the exo-embryo of course 

 corresponding to the normal internal apertures leading out of the 



