AMPfflBIANS 



239 



ments and reshufflings of their cell material 

 before they settle down for final differentia- 

 tion. These rearrangements can be compared 

 with the movements of gastrulation, al- 

 though, under the present conditions, they 



80b). Fragments containing the material of 

 the caudal somites and notochord frequently 

 produce merely epidermis which is some- 

 times associated with ill-defined connective 

 tissue. It appears that the latter represent 



a b 



Fig. 80. a, Differentiations in normal development (prospective significance) ; b, differentiations obtained 

 when small pieces of the prospective areas are cultured as explants. The results are projected back upon the 

 side-view of an early gastrula. (After Holtfreter, '36.) 



are bound to take an abnormal course (Holt- 

 freter, '38b, '39b; Schechtman, '42). 



If the differentiations are related to the 

 fate map, it becomes evident that the dorsal 

 blastoporal region has the properties of an 

 "embryonic field." Here are some relevant 

 data: 



1. A small piece from the presumptive 

 notochord not only gives rise to a notochord 

 but will also form somites and, frequently, 

 even various neural and epidermal structures. 

 The same array of tissues can be obtained 

 from pieces isolated from the area of the 

 presumptive somites (Fig. 81). This shows 

 that at the time of isolation the explanted 

 areas do not differ noticeably in their ten- 

 dencies of self-differentiation, all of them 

 producing a greater variety of tissues than 

 they would do when left undisturbed within 

 the embryo. 



The gradient nature of this "chorda-meso- 

 derm field" (Weiss, '39) is expressed in the 

 distinct decrease of chorda-mesoderm differ- 

 entiations at its periphery. The frequency of 

 regulative differentiations is highest in ex- 

 plants comprising the prechordal material 

 and the central portion of the prospective 

 chorda-mesoderm; it decreases toward the 

 animal pole and the lateroventral region of 

 the marginal zone (compare Figs. 80a and 



prospective skeletal muscle which could not 

 differentiate typically because it was not as- 

 sociated with notochord tissue. 



2. When attached to glass, a blastoporal 

 fragment tends to form an axial system ex- 



Fig. 81. An explanted piece of the prospective 

 somite region of an early gastrula differentiates 

 into an axial system consisting of median notochord 

 and spinal cord and bilaterally arranged myoblasts. 

 Notice anterior brainlike enlargement of the neural 

 tube, capped by epidermis and surrounded by mi- 

 grating mesenchyme and "giant cells" which seem 

 to represent cephalic mesectoderm. (After Holt- 

 freter, '38b.) 



hibiting cephalocaudal polarity and bilater- 

 ally symmetrical arrangement of its myo- 

 blasts (Fig. 81). This applies even to pieces 

 from the lateral blastoporal region. Hence, 

 one may ascribe to the chorda-mesoderm 



