464 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



but is, of course, inside out (Fig. 159, C; Fig. 160). All gradations between 

 complete exogastrulation and normal embryos occur. In partial exogas- 

 trulae evagination is followed by more or less invagination; and, if this 

 is sufficient to bring chorda-mesoderm beneath ectoderm, neural tissue 

 may develop, varying in form and size with form and size of invaginated 

 chorda-mesoderm and differentiating into the more posterior parts of the 

 neural tube, or only the caudal portion when only a small part, represent- 

 ing the posterior portion of the chorda-mesoderm, invaginates. Pieces of 



Epldfixmb 



Kopfmesoderm 

 Munddarm 



Fig. 159, A-C. — Exogastrulation in axolotl. A, B, directions of regional migrations; C, 

 diagrammatic longitudinal section, indicating differentiation of mesentoderm and lack of 

 differentiation of ectoderm. Parts indicated: coelom, intestine {Diinndarm) , pronephric 

 tubule {Nierenkanal), trunk muscles (Rumpfmuskel), notochord {Chorda), pharynx (Kienien- 

 darm), head mesoderm {Kopfmesoderm), oral entoderm {Munddarm), rectum {Enddarm), 

 epidermis (from Holtfreter, i933e, Biol. Zbl., 53, H. 7, 8, Thieme). 



ectoderm from early gastrulae, transplanted to the surface of the evagi- 

 nated chorda-mesoderm, differentiate according to the region of the in- 

 ductor on which they lie. On the head inductor they may develop into 

 brain, with eyes, olfactory pits, and ear vesicles; on the trunk inductor 

 they develop spinal cord; and at levels still farther posterior a tail may 

 develop, with growth of chorda-mesoderm into the ectoderm and induc- 

 tion of a neural tube. These exogastrulae are regarded by Holtfreter as 

 providing complete proof that there is no determination in the ectoderm 

 before invagination of the chorda-mesoderm and, consequently, that its 

 development is entirely the result of induction in the normal individual. 

 This conclusion ignores, or regards as erroneous, evidence from earlier 



