236 



Embryogenesis: Progressive Differentiation 



derm which eventually envelops it entirely. 

 The equatorial zone corresponding to the 

 blastoporal rim is successively constricted 

 until it is narrowed down to a thread which 

 connects the attenuated tail-end of the ento- 

 mesoderm with the empty bag of the stripped- 

 off ectoderm (Fig. 78c). The latter, thus de- 

 prived of its normal underlying tissues, proves 

 to be incapable of differentiating into any of 

 its normal derivatives, such as a neural sys- 



PARTIAL ORGANIZATION AS A RESULT 

 OF PARTIAL INVAGINATION 



The supposition that lack of an ectodermal 

 organization is caused by the absence of 

 subjacent entomesoderm is confirmed by 

 observations on partially exogastrulated em- 

 bryos. Varying with the concentration of 

 the ambient salt solution and with the time 

 of exposvire, one may obtain a continuous 



Gut epithel. Nofochord 



Fig. 78. Diagrammatic illustration of the morphogenetic movements (a) of normal gastrulation and (b) 

 of exogastrulation. c, Main differentiations in an exogastrulated embryo. (After Holtfreter, '33a.) 



tem, sense organs or specific epidermal struc- 

 tures. It merely develops into a mass of un- 

 specialized epidermis cells. It follows that 

 the entomesoderm contains determinative 

 factors necessary for any typical ectodermal 

 differentiation. 



The exogastrulated material, on the other 

 hand, may adopt the shape of an embryo and 

 differentiate quite normally in the total ab- 

 sence of ectodermal tissues (Fig. 78c). Thus 

 the displaced marginal zone develops into an 

 axial notochord associated with somites and 

 pronephroi, and furthermore into coelomic 

 cavities, head muscles, connective tissue, 

 blood, some cartilage, heart and smooth in- 

 testinal muscles. The latter two, though not 

 innervated, may perform rhythmic contrac- 

 tions. These mesodermal tissues are wrapped 

 into entoderm which differentiates into the 

 various kinds of glands and epithelia charac- 

 teristic of the normal intestinal system. How- 

 ever, the polarity of these epithelia is in- 

 verted, their secreting surface being turned 

 toward the external medium. 



series of gradations between complete exo- 

 gastrulae and normally invaginated embryos. 

 Figure 79 gives typical examples of such mal- 

 formations in R. pipiens, which in this case, 

 however, were obtained by a shock treatment 

 with alkali. This series differs from corre- 

 sponding ones in urodeles (Holtfreter, '33d) 

 mainly by the fact that the latter rarely ex- 

 hibit the various degrees of tail duplication 

 and spina bifida which resvilt in anurans 

 from a failure of the lateral primordia to join 

 in the dorsal midline (in contrast to the 

 urodeles, the prospective tail somites in the 

 anuran gastrula are located more laterally). 

 In all cases, however, the extent and type 

 of ectodermal organization can be related to 

 the amount and histological nature of the 

 entomesoderm that has invaginated. Thus 

 the appearance of spinal cord and fin 

 depends upon the presence of underlying 

 notochord and somites while the various 

 ectoderm structures of the head develop 

 only when cephalic entomesoderm has in- 

 vaginated. 



